2021-02-25 13:25:43

by Muchun Song

Subject: [PATCH v17 0/9] Free some vmemmap pages of HugeTLB page

Hi all,

This patch series will free some vmemmap pages(struct page structures)
associated with each hugetlbpage when preallocated to save memory.

In order to reduce the difficulty of the first version of code review.
From this version, we disable PMD/huge page mapping of vmemmap if this
feature was enabled. This accutualy eliminate a bunch of the complex code
doing page table manipulation. When this patch series is solid, we cam add
the code of vmemmap page table manipulation in the future.

The struct page structures (page structs) are used to describe a physical
page frame. By default, there is a one-to-one mapping from a page frame to
it's corresponding page struct.

The HugeTLB pages consist of multiple base page size pages and is supported
by many architectures. See hugetlbpage.rst in the Documentation directory
for more details. On the x86 architecture, HugeTLB pages of size 2MB and 1GB
are currently supported. Since the base page size on x86 is 4KB, a 2MB
HugeTLB page consists of 512 base pages and a 1GB HugeTLB page consists of
4096 base pages. For each base page, there is a corresponding page struct.

Within the HugeTLB subsystem, only the first 4 page structs are used to
contain unique information about a HugeTLB page. HUGETLB_CGROUP_MIN_ORDER
provides this upper limit. The only 'useful' information in the remaining
page structs is the compound_head field, and this field is the same for all
tail pages.

By removing redundant page structs for HugeTLB pages, memory can returned to
the buddy allocator for other uses.

When the system boot up, every 2M HugeTLB has 512 struct page structs which
size is 8 pages(sizeof(struct page) * 512 / PAGE_SIZE).

HugeTLB struct pages(8 pages) page frame(8 pages)
+-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+
| | | 0 | -------------> | 0 |
| | +-----------+ +-----------+
| | | 1 | -------------> | 1 |
| | +-----------+ +-----------+
| | | 2 | -------------> | 2 |
| | +-----------+ +-----------+
| | | 3 | -------------> | 3 |
| | +-----------+ +-----------+
| | | 4 | -------------> | 4 |
| 2MB | +-----------+ +-----------+
| | | 5 | -------------> | 5 |
| | +-----------+ +-----------+
| | | 6 | -------------> | 6 |
| | +-----------+ +-----------+
| | | 7 | -------------> | 7 |
| | +-----------+ +-----------+
| |
| |
| |
+-----------+

The value of page->compound_head is the same for all tail pages. The first
page of page structs (page 0) associated with the HugeTLB page contains the 4
page structs necessary to describe the HugeTLB. The only use of the remaining
pages of page structs (page 1 to page 7) is to point to page->compound_head.
Therefore, we can remap pages 2 to 7 to page 1. Only 2 pages of page structs
will be used for each HugeTLB page. This will allow us to free the remaining
6 pages to the buddy allocator.

Here is how things look after remapping.

HugeTLB struct pages(8 pages) page frame(8 pages)
+-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+
| | | 0 | -------------> | 0 |
| | +-----------+ +-----------+
| | | 1 | -------------> | 1 |
| | +-----------+ +-----------+
| | | 2 | ----------------^ ^ ^ ^ ^ ^
| | +-----------+ | | | | |
| | | 3 | ------------------+ | | | |
| | +-----------+ | | | |
| | | 4 | --------------------+ | | |
| 2MB | +-----------+ | | |
| | | 5 | ----------------------+ | |
| | +-----------+ | |
| | | 6 | ------------------------+ |
| | +-----------+ |
| | | 7 | --------------------------+
| | +-----------+
| |
| |
| |
+-----------+

When a HugeTLB is freed to the buddy system, we should allocate 6 pages for
vmemmap pages and restore the previous mapping relationship.

Apart from 2MB HugeTLB page, we also have 1GB HugeTLB page. It is similar
to the 2MB HugeTLB page. We also can use this approach to free the vmemmap
pages.

In this case, for the 1GB HugeTLB page, we can save 4094 pages. This is a
very substantial gain. On our server, run some SPDK/QEMU applications which
will use 1024GB hugetlbpage. With this feature enabled, we can save ~16GB
(1G hugepage)/~12GB (2MB hugepage) memory.

Because there are vmemmap page tables reconstruction on the freeing/allocating
path, it increases some overhead. Here are some overhead analysis.

1) Allocating 10240 2MB hugetlb pages.

a) With this patch series applied:
# time echo 10240 > /proc/sys/vm/nr_hugepages

real 0m0.166s
user 0m0.000s
sys 0m0.166s

# bpftrace -e 'kprobe:alloc_fresh_huge_page { @start[tid] = nsecs; }
kretprobe:alloc_fresh_huge_page /@start[tid]/ { @latency = hist(nsecs -
@start[tid]); delete(@start[tid]); }'
Attaching 2 probes...

@latency:
[8K, 16K) 5476 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[16K, 32K) 4760 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ |
[32K, 64K) 4 | |

b) Without this patch series:
# time echo 10240 > /proc/sys/vm/nr_hugepages

real 0m0.067s
user 0m0.000s
sys 0m0.067s

# bpftrace -e 'kprobe:alloc_fresh_huge_page { @start[tid] = nsecs; }
kretprobe:alloc_fresh_huge_page /@start[tid]/ { @latency = hist(nsecs -
@start[tid]); delete(@start[tid]); }'
Attaching 2 probes...

@latency:
[4K, 8K) 10147 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[8K, 16K) 93 | |

Summarize: this feature is about ~2x slower than before.

2) Freeing 10240 2MB hugetlb pages.

a) With this patch series applied:
# time echo 0 > /proc/sys/vm/nr_hugepages

real 0m0.213s
user 0m0.000s
sys 0m0.213s

# bpftrace -e 'kprobe:free_pool_huge_page { @start[tid] = nsecs; }
kretprobe:free_pool_huge_page /@start[tid]/ { @latency = hist(nsecs -
@start[tid]); delete(@start[tid]); }'
Attaching 2 probes...

@latency:
[8K, 16K) 6 | |
[16K, 32K) 10227 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[32K, 64K) 7 | |

b) Without this patch series:
# time echo 0 > /proc/sys/vm/nr_hugepages

real 0m0.081s
user 0m0.000s
sys 0m0.081s

# bpftrace -e 'kprobe:free_pool_huge_page { @start[tid] = nsecs; }
kretprobe:free_pool_huge_page /@start[tid]/ { @latency = hist(nsecs -
@start[tid]); delete(@start[tid]); }'
Attaching 2 probes...

@latency:
[4K, 8K) 6805 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[8K, 16K) 3427 |@@@@@@@@@@@@@@@@@@@@@@@@@@ |
[16K, 32K) 8 | |

Summarize: The overhead of __free_hugepage is about ~2-3x slower than before.

Although the overhead has increased, the overhead is not significant. Like Mike
said, "However, remember that the majority of use cases create hugetlb pages at
or shortly after boot time and add them to the pool. So, additional overhead is
at pool creation time. There is no change to 'normal run time' operations of
getting a page from or returning a page to the pool (think page fault/unmap)".

Despite the overhead and in addition to the memory gains from this series. The
following data is obtained by Joao Martins. Very thanks to his effort.

There's an additional benefit which is page (un)pinners will see an improvement
and Joao presumes because there are fewer memmap pages and thus the tail/head
pages are staying in cache more often.

Out of the box Joao saw (when comparing linux-next against linux-next + this series)
with gup_test and pinning a 16G hugetlb file (with 1G pages):

get_user_pages(): ~32k -> ~9k
unpin_user_pages(): ~75k -> ~70k

Usually any tight loop fetching compound_head(), or reading tail pages data (e.g.
compound_head) benefit a lot. There's some unpinning inefficiencies Joao was
fixing[0], but with that in added it shows even more:

unpin_user_pages(): ~27k -> ~3.8k

[0] https://lore.kernel.org/linux-mm/[email protected]/

Todo:
- Free all of the tail vmemmap pages
Now for the 2MB HugrTLB page, we only free 6 vmemmap pages. we really can
free 7 vmemmap pages. In this case, we can see 8 of the 512 struct page
structures has beed set PG_head flag. If we can adjust compound_head()
slightly and make compound_head() return the real head struct page when
the parameter is the tail struct page but with PG_head flag set.

In order to make the code evolution route clearer. This feature can can be
a separate patch after this patchset is solid.

- Support for other architectures (e.g. aarch64).
- Enable PMD/huge page mapping of vmemmap even if this feature was enabled.

Changelog in v16 -> v17:
- Fix issues suggested by Mike and Oscar.
- Update commit log suggested by Michal.

Thanks to Mike, David H and Michal's suggestions and review.

Changelog in v15 -> v16:
- Use GFP_KERNEL to allocate vmemmap pages.

Thanks to Mike, David H and Michal's suggestions.

Changelog in v14 -> v15:
- Fix some issues suggested by Oscar. Thanks to Oscar.
- Add numbers which Joao Martins tested to cover letter. Thanks to his effort.

Changelog in v13 -> v14:
- Refuse to free the HugeTLB page when the system is under memory pressure.
- Use GFP_ATOMIC to allocate vmemmap pages instead of GFP_KERNEL.
- Rebase to linux-next 20210202.
- Fix and add some comments for vmemmap_remap_free().

Thanks to Oscar, Mike, David H and David R's suggestions and review.

Changelog in v12 -> v13:
- Remove VM_WARN_ON_PAGE macro.
- Add more comments in vmemmap_pte_range() and vmemmap_remap_free().

Thanks to Oscar and Mike's suggestions and review.

Changelog in v11 -> v12:
- Move VM_WARN_ON_PAGE to a separate patch.
- Call __free_hugepage() with hugetlb_lock (See patch #5.) to serialize
with dissolve_free_huge_page(). It is to prepare for patch #9.
- Introduce PageHugeInflight. See patch #9.

Changelog in v10 -> v11:
- Fix compiler error when !CONFIG_HUGETLB_PAGE_FREE_VMEMMAP.
- Rework some comments and commit changes.
- Rework vmemmap_remap_free() to 3 parameters.

Thanks to Oscar and Mike's suggestions and review.

Changelog in v9 -> v10:
- Fix a bug in patch #11. Thanks to Oscar for pointing that out.
- Rework some commit log or comments. Thanks Mike and Oscar for the suggestions.
- Drop VMEMMAP_TAIL_PAGE_REUSE in the patch #3.

Thank you very much Mike and Oscar for reviewing the code.

Changelog in v8 -> v9:
- Rework some code. Very thanks to Oscar.
- Put all the non-hugetlb vmemmap functions under sparsemem-vmemmap.c.

Changelog in v7 -> v8:
- Adjust the order of patches.

Very thanks to David and Oscar. Your suggestions are very valuable.

Changelog in v6 -> v7:
- Rebase to linux-next 20201130
- Do not use basepage mapping for vmemmap when this feature is disabled.
- Rework some patchs.
[PATCH v6 08/16] mm/hugetlb: Free the vmemmap pages associated with each hugetlb page
[PATCH v6 10/16] mm/hugetlb: Allocate the vmemmap pages associated with each hugetlb page

Thanks to Oscar and Barry.

Changelog in v5 -> v6:
- Disable PMD/huge page mapping of vmemmap if this feature was enabled.
- Simplify the first version code.

Changelog in v4 -> v5:
- Rework somme comments and code in the [PATCH v4 04/21] and [PATCH v4 05/21].

Thanks to Mike and Oscar's suggestions.

Changelog in v3 -> v4:
- Move all the vmemmap functions to hugetlb_vmemmap.c.
- Make the CONFIG_HUGETLB_PAGE_FREE_VMEMMAP default to y, if we want to
disable this feature, we should disable it by a boot/kernel command line.
- Remove vmemmap_pgtable_{init, deposit, withdraw}() helper functions.
- Initialize page table lock for vmemmap through core_initcall mechanism.

Thanks for Mike and Oscar's suggestions.

Changelog in v2 -> v3:
- Rename some helps function name. Thanks Mike.
- Rework some code. Thanks Mike and Oscar.
- Remap the tail vmemmap page with PAGE_KERNEL_RO instead of PAGE_KERNEL.
Thanks Matthew.
- Add some overhead analysis in the cover letter.
- Use vmemap pmd table lock instead of a hugetlb specific global lock.

Changelog in v1 -> v2:
- Fix do not call dissolve_compound_page in alloc_huge_page_vmemmap().
- Fix some typo and code style problems.
- Remove unused handle_vmemmap_fault().
- Merge some commits to one commit suggested by Mike.

Muchun Song (9):
mm: memory_hotplug: factor out bootmem core functions to
bootmem_info.c
mm: hugetlb: introduce a new config HUGETLB_PAGE_FREE_VMEMMAP
mm: hugetlb: free the vmemmap pages associated with each HugeTLB page
mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page
mm: hugetlb: set the PageHWPoison to the raw error page
mm: hugetlb: add a kernel parameter hugetlb_free_vmemmap
mm: hugetlb: introduce nr_free_vmemmap_pages in the struct hstate
mm: hugetlb: gather discrete indexes of tail page
mm: hugetlb: optimize the code with the help of the compiler

Documentation/admin-guide/kernel-parameters.txt | 14 ++
Documentation/admin-guide/mm/hugetlbpage.rst | 11 +
arch/x86/mm/init_64.c | 13 +-
fs/Kconfig | 6 +
include/linux/bootmem_info.h | 65 ++++++
include/linux/hugetlb.h | 47 +++-
include/linux/hugetlb_cgroup.h | 19 +-
include/linux/memory_hotplug.h | 27 ---
include/linux/mm.h | 5 +
mm/Makefile | 2 +
mm/bootmem_info.c | 124 ++++++++++
mm/hugetlb.c | 176 +++++++++++---
mm/hugetlb_vmemmap.c | 293 ++++++++++++++++++++++++
mm/hugetlb_vmemmap.h | 51 +++++
mm/memory_hotplug.c | 116 ----------
mm/sparse-vmemmap.c | 280 ++++++++++++++++++++++
mm/sparse.c | 1 +
17 files changed, 1062 insertions(+), 188 deletions(-)
create mode 100644 include/linux/bootmem_info.h
create mode 100644 mm/bootmem_info.c
create mode 100644 mm/hugetlb_vmemmap.c
create mode 100644 mm/hugetlb_vmemmap.h

--
2.11.0

2021-02-25 13:26:02

by Muchun Song

Subject: [PATCH v17 1/9] mm: memory_hotplug: factor out bootmem core functions to bootmem_info.c

Move bootmem info registration common API to individual bootmem_info.c.
And we will use {get,put}_page_bootmem() to initialize the page for the
vmemmap pages or free the vmemmap pages to buddy in the later patch.
So move them out of CONFIG_MEMORY_HOTPLUG_SPARSE. This is just code
movement without any functional change.

Signed-off-by: Muchun Song <[email protected]>
Acked-by: Mike Kravetz <[email protected]>
Reviewed-by: Oscar Salvador <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Reviewed-by: Miaohe Lin <[email protected]>
---
arch/x86/mm/init_64.c | 3 +-
include/linux/bootmem_info.h | 40 +++++++++++++
include/linux/memory_hotplug.h | 27 ---------
mm/Makefile | 1 +
mm/bootmem_info.c | 124 +++++++++++++++++++++++++++++++++++++++++
mm/memory_hotplug.c | 116 --------------------------------------
mm/sparse.c | 1 +
7 files changed, 168 insertions(+), 144 deletions(-)
create mode 100644 include/linux/bootmem_info.h
create mode 100644 mm/bootmem_info.c

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index b5a3fa4033d3..0a45f062826e 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -33,6 +33,7 @@
#include <linux/nmi.h>
#include <linux/gfp.h>
#include <linux/kcore.h>
+#include <linux/bootmem_info.h>

#include <asm/processor.h>
#include <asm/bios_ebda.h>
@@ -1571,7 +1572,7 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
return err;
}

-#if defined(CONFIG_MEMORY_HOTPLUG_SPARSE) && defined(CONFIG_HAVE_BOOTMEM_INFO_NODE)
+#ifdef CONFIG_HAVE_BOOTMEM_INFO_NODE
void register_page_bootmem_memmap(unsigned long section_nr,
struct page *start_page, unsigned long nr_pages)
{
diff --git a/include/linux/bootmem_info.h b/include/linux/bootmem_info.h
new file mode 100644
index 000000000000..4ed6dee1adc9
--- /dev/null
+++ b/include/linux/bootmem_info.h
@@ -0,0 +1,40 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __LINUX_BOOTMEM_INFO_H
+#define __LINUX_BOOTMEM_INFO_H
+
+#include <linux/mmzone.h>
+
+/*
+ * Types for free bootmem stored in page->lru.next. These have to be in
+ * some random range in unsigned long space for debugging purposes.
+ */
+enum {
+ MEMORY_HOTPLUG_MIN_BOOTMEM_TYPE = 12,
+ SECTION_INFO = MEMORY_HOTPLUG_MIN_BOOTMEM_TYPE,
+ MIX_SECTION_INFO,
+ NODE_INFO,
+ MEMORY_HOTPLUG_MAX_BOOTMEM_TYPE = NODE_INFO,
+};
+
+#ifdef CONFIG_HAVE_BOOTMEM_INFO_NODE
+void __init register_page_bootmem_info_node(struct pglist_data *pgdat);
+
+void get_page_bootmem(unsigned long info, struct page *page,
+ unsigned long type);
+void put_page_bootmem(struct page *page);
+#else
+static inline void register_page_bootmem_info_node(struct pglist_data *pgdat)
+{
+}
+
+static inline void put_page_bootmem(struct page *page)
+{
+}
+
+static inline void get_page_bootmem(unsigned long info, struct page *page,
+ unsigned long type)
+{
+}
+#endif
+
+#endif /* __LINUX_BOOTMEM_INFO_H */
diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
index 7288aa5ef73b..96659a8b9d02 100644
--- a/include/linux/memory_hotplug.h
+++ b/include/linux/memory_hotplug.h
@@ -18,18 +18,6 @@ struct vmem_altmap;
#ifdef CONFIG_MEMORY_HOTPLUG
struct page *pfn_to_online_page(unsigned long pfn);

-/*
- * Types for free bootmem stored in page->lru.next. These have to be in
- * some random range in unsigned long space for debugging purposes.
- */
-enum {
- MEMORY_HOTPLUG_MIN_BOOTMEM_TYPE = 12,
- SECTION_INFO = MEMORY_HOTPLUG_MIN_BOOTMEM_TYPE,
- MIX_SECTION_INFO,
- NODE_INFO,
- MEMORY_HOTPLUG_MAX_BOOTMEM_TYPE = NODE_INFO,
-};
-
/* Types for control the zone type of onlined and offlined memory */
enum {
/* Offline the memory. */
@@ -210,17 +198,6 @@ static inline void arch_refresh_nodedata(int nid, pg_data_t *pgdat)
#endif /* CONFIG_NUMA */
#endif /* CONFIG_HAVE_ARCH_NODEDATA_EXTENSION */

-#ifdef CONFIG_HAVE_BOOTMEM_INFO_NODE
-extern void __init register_page_bootmem_info_node(struct pglist_data *pgdat);
-#else
-static inline void register_page_bootmem_info_node(struct pglist_data *pgdat)
-{
-}
-#endif
-extern void put_page_bootmem(struct page *page);
-extern void get_page_bootmem(unsigned long ingo, struct page *page,
- unsigned long type);
-
void get_online_mems(void);
void put_online_mems(void);

@@ -248,10 +225,6 @@ static inline void zone_span_writelock(struct zone *zone) {}
static inline void zone_span_writeunlock(struct zone *zone) {}
static inline void zone_seqlock_init(struct zone *zone) {}

-static inline void register_page_bootmem_info_node(struct pglist_data *pgdat)
-{
-}
-
static inline int try_online_node(int nid)
{
return 0;
diff --git a/mm/Makefile b/mm/Makefile
index 72227b24a616..daabf86d7da8 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -83,6 +83,7 @@ obj-$(CONFIG_SLUB) += slub.o
obj-$(CONFIG_KASAN) += kasan/
obj-$(CONFIG_KFENCE) += kfence/
obj-$(CONFIG_FAILSLAB) += failslab.o
+obj-$(CONFIG_HAVE_BOOTMEM_INFO_NODE) += bootmem_info.o
obj-$(CONFIG_MEMORY_HOTPLUG) += memory_hotplug.o
obj-$(CONFIG_MEMTEST) += memtest.o
obj-$(CONFIG_MIGRATION) += migrate.o
diff --git a/mm/bootmem_info.c b/mm/bootmem_info.c
new file mode 100644
index 000000000000..fcab5a3f8cc0
--- /dev/null
+++ b/mm/bootmem_info.c
@@ -0,0 +1,124 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * linux/mm/bootmem_info.c
+ *
+ * Copyright (C)
+ */
+#include <linux/mm.h>
+#include <linux/compiler.h>
+#include <linux/memblock.h>
+#include <linux/bootmem_info.h>
+#include <linux/memory_hotplug.h>
+
+void get_page_bootmem(unsigned long info, struct page *page, unsigned long type)
+{
+ page->freelist = (void *)type;
+ SetPagePrivate(page);
+ set_page_private(page, info);
+ page_ref_inc(page);
+}
+
+void put_page_bootmem(struct page *page)
+{
+ unsigned long type;
+
+ type = (unsigned long) page->freelist;
+ BUG_ON(type < MEMORY_HOTPLUG_MIN_BOOTMEM_TYPE ||
+ type > MEMORY_HOTPLUG_MAX_BOOTMEM_TYPE);
+
+ if (page_ref_dec_return(page) == 1) {
+ page->freelist = NULL;
+ ClearPagePrivate(page);
+ set_page_private(page, 0);
+ INIT_LIST_HEAD(&page->lru);
+ free_reserved_page(page);
+ }
+}
+
+#ifndef CONFIG_SPARSEMEM_VMEMMAP
+static void register_page_bootmem_info_section(unsigned long start_pfn)
+{
+ unsigned long mapsize, section_nr, i;
+ struct mem_section *ms;
+ struct page *page, *memmap;
+ struct mem_section_usage *usage;
+
+ section_nr = pfn_to_section_nr(start_pfn);
+ ms = __nr_to_section(section_nr);
+
+ /* Get section's memmap address */
+ memmap = sparse_decode_mem_map(ms->section_mem_map, section_nr);
+
+ /*
+ * Get page for the memmap's phys address
+ * XXX: need more consideration for sparse_vmemmap...
+ */
+ page = virt_to_page(memmap);
+ mapsize = sizeof(struct page) * PAGES_PER_SECTION;
+ mapsize = PAGE_ALIGN(mapsize) >> PAGE_SHIFT;
+
+ /* remember memmap's page */
+ for (i = 0; i < mapsize; i++, page++)
+ get_page_bootmem(section_nr, page, SECTION_INFO);
+
+ usage = ms->usage;
+ page = virt_to_page(usage);
+
+ mapsize = PAGE_ALIGN(mem_section_usage_size()) >> PAGE_SHIFT;
+
+ for (i = 0; i < mapsize; i++, page++)
+ get_page_bootmem(section_nr, page, MIX_SECTION_INFO);
+
+}
+#else /* CONFIG_SPARSEMEM_VMEMMAP */
+static void register_page_bootmem_info_section(unsigned long start_pfn)
+{
+ unsigned long mapsize, section_nr, i;
+ struct mem_section *ms;
+ struct page *page, *memmap;
+ struct mem_section_usage *usage;
+
+ section_nr = pfn_to_section_nr(start_pfn);
+ ms = __nr_to_section(section_nr);
+
+ memmap = sparse_decode_mem_map(ms->section_mem_map, section_nr);
+
+ register_page_bootmem_memmap(section_nr, memmap, PAGES_PER_SECTION);
+
+ usage = ms->usage;
+ page = virt_to_page(usage);
+
+ mapsize = PAGE_ALIGN(mem_section_usage_size()) >> PAGE_SHIFT;
+
+ for (i = 0; i < mapsize; i++, page++)
+ get_page_bootmem(section_nr, page, MIX_SECTION_INFO);
+}
+#endif /* !CONFIG_SPARSEMEM_VMEMMAP */
+
+void __init register_page_bootmem_info_node(struct pglist_data *pgdat)
+{
+ unsigned long i, pfn, end_pfn, nr_pages;
+ int node = pgdat->node_id;
+ struct page *page;
+
+ nr_pages = PAGE_ALIGN(sizeof(struct pglist_data)) >> PAGE_SHIFT;
+ page = virt_to_page(pgdat);
+
+ for (i = 0; i < nr_pages; i++, page++)
+ get_page_bootmem(node, page, NODE_INFO);
+
+ pfn = pgdat->node_start_pfn;
+ end_pfn = pgdat_end_pfn(pgdat);
+
+ /* register section info */
+ for (; pfn < end_pfn; pfn += PAGES_PER_SECTION) {
+ /*
+ * Some platforms can assign the same pfn to multiple nodes - on
+ * node0 as well as nodeN. To avoid registering a pfn against
+ * multiple nodes we check that this pfn does not already
+ * reside in some other nodes.
+ */
+ if (pfn_valid(pfn) && (early_pfn_to_nid(pfn) == node))
+ register_page_bootmem_info_section(pfn);
+ }
+}
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 5ba51a8bdaeb..a2a72b617040 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -144,122 +144,6 @@ static void release_memory_resource(struct resource *res)
}

#ifdef CONFIG_MEMORY_HOTPLUG_SPARSE
-void get_page_bootmem(unsigned long info, struct page *page,
- unsigned long type)
-{
- page->freelist = (void *)type;
- SetPagePrivate(page);
- set_page_private(page, info);
- page_ref_inc(page);
-}
-
-void put_page_bootmem(struct page *page)
-{
- unsigned long type;
-
- type = (unsigned long) page->freelist;
- BUG_ON(type < MEMORY_HOTPLUG_MIN_BOOTMEM_TYPE ||
- type > MEMORY_HOTPLUG_MAX_BOOTMEM_TYPE);
-
- if (page_ref_dec_return(page) == 1) {
- page->freelist = NULL;
- ClearPagePrivate(page);
- set_page_private(page, 0);
- INIT_LIST_HEAD(&page->lru);
- free_reserved_page(page);
- }
-}
-
-#ifdef CONFIG_HAVE_BOOTMEM_INFO_NODE
-#ifndef CONFIG_SPARSEMEM_VMEMMAP
-static void register_page_bootmem_info_section(unsigned long start_pfn)
-{
- unsigned long mapsize, section_nr, i;
- struct mem_section *ms;
- struct page *page, *memmap;
- struct mem_section_usage *usage;
-
- section_nr = pfn_to_section_nr(start_pfn);
- ms = __nr_to_section(section_nr);
-
- /* Get section's memmap address */
- memmap = sparse_decode_mem_map(ms->section_mem_map, section_nr);
-
- /*
- * Get page for the memmap's phys address
- * XXX: need more consideration for sparse_vmemmap...
- */
- page = virt_to_page(memmap);
- mapsize = sizeof(struct page) * PAGES_PER_SECTION;
- mapsize = PAGE_ALIGN(mapsize) >> PAGE_SHIFT;
-
- /* remember memmap's page */
- for (i = 0; i < mapsize; i++, page++)
- get_page_bootmem(section_nr, page, SECTION_INFO);
-
- usage = ms->usage;
- page = virt_to_page(usage);
-
- mapsize = PAGE_ALIGN(mem_section_usage_size()) >> PAGE_SHIFT;
-
- for (i = 0; i < mapsize; i++, page++)
- get_page_bootmem(section_nr, page, MIX_SECTION_INFO);
-
-}
-#else /* CONFIG_SPARSEMEM_VMEMMAP */
-static void register_page_bootmem_info_section(unsigned long start_pfn)
-{
- unsigned long mapsize, section_nr, i;
- struct mem_section *ms;
- struct page *page, *memmap;
- struct mem_section_usage *usage;
-
- section_nr = pfn_to_section_nr(start_pfn);
- ms = __nr_to_section(section_nr);
-
- memmap = sparse_decode_mem_map(ms->section_mem_map, section_nr);
-
- register_page_bootmem_memmap(section_nr, memmap, PAGES_PER_SECTION);
-
- usage = ms->usage;
- page = virt_to_page(usage);
-
- mapsize = PAGE_ALIGN(mem_section_usage_size()) >> PAGE_SHIFT;
-
- for (i = 0; i < mapsize; i++, page++)
- get_page_bootmem(section_nr, page, MIX_SECTION_INFO);
-}
-#endif /* !CONFIG_SPARSEMEM_VMEMMAP */
-
-void __init register_page_bootmem_info_node(struct pglist_data *pgdat)
-{
- unsigned long i, pfn, end_pfn, nr_pages;
- int node = pgdat->node_id;
- struct page *page;
-
- nr_pages = PAGE_ALIGN(sizeof(struct pglist_data)) >> PAGE_SHIFT;
- page = virt_to_page(pgdat);
-
- for (i = 0; i < nr_pages; i++, page++)
- get_page_bootmem(node, page, NODE_INFO);
-
- pfn = pgdat->node_start_pfn;
- end_pfn = pgdat_end_pfn(pgdat);
-
- /* register section info */
- for (; pfn < end_pfn; pfn += PAGES_PER_SECTION) {
- /*
- * Some platforms can assign the same pfn to multiple nodes - on
- * node0 as well as nodeN. To avoid registering a pfn against
- * multiple nodes we check that this pfn does not already
- * reside in some other nodes.
- */
- if (pfn_valid(pfn) && (early_pfn_to_nid(pfn) == node))
- register_page_bootmem_info_section(pfn);
- }
-}
-#endif /* CONFIG_HAVE_BOOTMEM_INFO_NODE */
-
static int check_pfn_span(unsigned long pfn, unsigned long nr_pages,
const char *reason)
{
diff --git a/mm/sparse.c b/mm/sparse.c
index 7bd23f9d6cef..87676bf3af40 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -13,6 +13,7 @@
#include <linux/vmalloc.h>
#include <linux/swap.h>
#include <linux/swapops.h>
+#include <linux/bootmem_info.h>

#include "internal.h"
#include <asm/dma.h>
--
2.11.0

2021-02-25 13:26:40

by Muchun Song

Subject: [PATCH v17 2/9] mm: hugetlb: introduce a new config HUGETLB_PAGE_FREE_VMEMMAP

The option HUGETLB_PAGE_FREE_VMEMMAP allows for the freeing of
some vmemmap pages associated with pre-allocated HugeTLB pages.
For example, on X86_64 6 vmemmap pages of size 4KB each can be
saved for each 2MB HugeTLB page. 4094 vmemmap pages of size 4KB
each can be saved for each 1GB HugeTLB page.

When a HugeTLB page is allocated or freed, the vmemmap array
representing the range associated with the page will need to be
remapped. When a page is allocated, vmemmap pages are freed
after remapping. When a page is freed, previously discarded
vmemmap pages must be allocated before remapping.

The config option is introduced early so that supporting code
can be written to depend on the option. The initial version of
the code only provides support for x86-64.

Like other code which frees vmemmap, this config option depends on
HAVE_BOOTMEM_INFO_NODE. The routine register_page_bootmem_info() is
used to register bootmem info. Therefore, make sure
register_page_bootmem_info is enabled if HUGETLB_PAGE_FREE_VMEMMAP
is defined.

Signed-off-by: Muchun Song <[email protected]>
Reviewed-by: Oscar Salvador <[email protected]>
Acked-by: Mike Kravetz <[email protected]>
Reviewed-by: Miaohe Lin <[email protected]>
---
arch/x86/mm/init_64.c | 2 +-
fs/Kconfig | 6 ++++++
2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index 0a45f062826e..0435bee2e172 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -1225,7 +1225,7 @@ static struct kcore_list kcore_vsyscall;

static void __init register_page_bootmem_info(void)
{
-#ifdef CONFIG_NUMA
+#if defined(CONFIG_NUMA) || defined(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP)
int i;

for_each_online_node(i)
diff --git a/fs/Kconfig b/fs/Kconfig
index eccbcf1e3f2e..b5dcc68aab25 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -237,6 +237,12 @@ config HUGETLBFS
config HUGETLB_PAGE
def_bool HUGETLBFS

+config HUGETLB_PAGE_FREE_VMEMMAP
+ def_bool HUGETLB_PAGE
+ depends on X86_64
+ depends on SPARSEMEM_VMEMMAP
+ depends on HAVE_BOOTMEM_INFO_NODE
+
config MEMFD_CREATE
def_bool TMPFS || HUGETLBFS

--
2.11.0

2021-02-25 13:27:52

by Muchun Song

Subject: [PATCH v17 4/9] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page

When we free a HugeTLB page to the buddy allocator, we should allocate
the vmemmap pages associated with it. But we may cannot allocate vmemmap
pages when the system is under memory pressure, in this case, we just
refuse to free the HugeTLB page instead of looping forever trying to
allocate the pages. This changes some behavior (list below) on some
corner cases.

1) Failing to free a huge page triggered by the user (decrease nr_pages).

Need try again later by the user.

2) Failing to free a surplus huge page when freed by the application.

Try again later when freeing a huge page next time.

3) Failing to dissolve a free huge page on ZONE_MOVABLE via
offline_pages().

This is a bit unfortunate if we have plenty of ZONE_MOVABLE memory
but are low on kernel memory. For example, migration of huge pages
would still work, however, dissolving the free page does not work.
This is a corner cases. When the system is that much under memory
pressure, offlining/unplug can be expected to fail. This is
unfortunate because it prevents from the memory offlining which
shouldn't happen for movable zones. People depending on the memory
hotplug and movable zone should carefuly consider whether savings
on unmovable memory are worth losing their hotplug functionality
in some situations.

4) Failing to dissolve a huge page on CMA/ZONE_MOVABLE via
alloc_contig_range() - once we have that handling in place. Mainly
affects CMA and virtio-mem.

Similar to 3). virito-mem will handle migration errors gracefully.
CMA might be able to fallback on other free areas within the CMA
region.

Vmemmap pages are allocated from the page freeing context. In order for
those allocations to be not disruptive (e.g. trigger oom killer)
__GFP_NORETRY is used. hugetlb_lock is dropped for the allocation
because a non sleeping allocation would be too fragile and it could fail
too easily under memory pressure. GFP_ATOMIC or other modes to access
memory reserves is not used because we want to prevent consuming
reserves under heavy hugetlb freeing.

Signed-off-by: Muchun Song <[email protected]>
---
Documentation/admin-guide/mm/hugetlbpage.rst | 8 +++
include/linux/mm.h | 2 +
mm/hugetlb.c | 92 +++++++++++++++++++++-------
mm/hugetlb_vmemmap.c | 32 ++++++----
mm/hugetlb_vmemmap.h | 23 +++++++
mm/sparse-vmemmap.c | 75 ++++++++++++++++++++++-
6 files changed, 197 insertions(+), 35 deletions(-)

diff --git a/Documentation/admin-guide/mm/hugetlbpage.rst b/Documentation/admin-guide/mm/hugetlbpage.rst
index f7b1c7462991..6988895d09a8 100644
--- a/Documentation/admin-guide/mm/hugetlbpage.rst
+++ b/Documentation/admin-guide/mm/hugetlbpage.rst
@@ -60,6 +60,10 @@ HugePages_Surp
the pool above the value in ``/proc/sys/vm/nr_hugepages``. The
maximum number of surplus huge pages is controlled by
``/proc/sys/vm/nr_overcommit_hugepages``.
+ Note: When the feature of freeing unused vmemmap pages associated
+ with each hugetlb page is enabled, the number of surplus huge pages
+ may be temporarily larger than the maximum number of surplus huge
+ pages when the system is under memory pressure.
Hugepagesize
is the default hugepage size (in Kb).
Hugetlb
@@ -80,6 +84,10 @@ returned to the huge page pool when freed by a task. A user with root
privileges can dynamically allocate more or free some persistent huge pages
by increasing or decreasing the value of ``nr_hugepages``.

+Note: When the feature of freeing unused vmemmap pages associated with each
+hugetlb page is enabled, we can fail to free the huge pages triggered by
+the user when ths system is under memory pressure. Please try again later.
+
Pages that are used as huge pages are reserved inside the kernel and cannot
be used for other purposes. Huge pages cannot be swapped out under
memory pressure.
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 4ddfc31f21c6..77693c944a36 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2973,6 +2973,8 @@ static inline void print_vma_addr(char *prefix, unsigned long rip)

void vmemmap_remap_free(unsigned long start, unsigned long end,
unsigned long reuse);
+int vmemmap_remap_alloc(unsigned long start, unsigned long end,
+ unsigned long reuse, gfp_t gfp_mask);

void *sparse_buffer_alloc(unsigned long size);
struct page * __populate_section_memmap(unsigned long pfn,
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 43fed6785322..b6e4e3f31ad2 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1304,16 +1304,59 @@ static inline void destroy_compound_gigantic_page(struct page *page,
unsigned int order) { }
#endif

-static void update_and_free_page(struct hstate *h, struct page *page)
+static int update_and_free_page(struct hstate *h, struct page *page)
+ __releases(&hugetlb_lock) __acquires(&hugetlb_lock)
{
int i;
struct page *subpage = page;
+ int nid = page_to_nid(page);

if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
- return;
+ return 0;

h->nr_huge_pages--;
- h->nr_huge_pages_node[page_to_nid(page)]--;
+ h->nr_huge_pages_node[nid]--;
+ VM_BUG_ON_PAGE(hugetlb_cgroup_from_page(page), page);
+ VM_BUG_ON_PAGE(hugetlb_cgroup_from_page_rsvd(page), page);
+ set_page_refcounted(page);
+ set_compound_page_dtor(page, NULL_COMPOUND_DTOR);
+
+ /*
+ * If the vmemmap pages associated with the HugeTLB page can be
+ * optimized or the page is gigantic, we might block in
+ * alloc_huge_page_vmemmap() or free_gigantic_page(). In both
+ * cases, drop the hugetlb_lock.
+ */
+ if (free_vmemmap_pages_per_hpage(h) || hstate_is_gigantic(h))
+ spin_unlock(&hugetlb_lock);
+
+ if (alloc_huge_page_vmemmap(h, page)) {
+ spin_lock(&hugetlb_lock);
+ INIT_LIST_HEAD(&page->lru);
+ set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
+ h->nr_huge_pages++;
+ h->nr_huge_pages_node[nid]++;
+
+ /*
+ * If we cannot allocate vmemmap pages, just refuse to free the
+ * page and put the page back on the hugetlb free list and treat
+ * as a surplus page.
+ */
+ h->surplus_huge_pages++;
+ h->surplus_huge_pages_node[nid]++;
+
+ /*
+ * The refcount can be perfectly increased by memory-failure or
+ * soft_offline handlers.
+ */
+ if (likely(put_page_testzero(page))) {
+ arch_clear_hugepage_flags(page);
+ enqueue_huge_page(h, page);
+ }
+
+ return -ENOMEM;
+ }
+
for (i = 0; i < pages_per_huge_page(h);
i++, subpage = mem_map_next(subpage, page, i)) {
subpage->flags &= ~(1 << PG_locked | 1 << PG_error |
@@ -1321,22 +1364,18 @@ static void update_and_free_page(struct hstate *h, struct page *page)
1 << PG_active | 1 << PG_private |
1 << PG_writeback);
}
- VM_BUG_ON_PAGE(hugetlb_cgroup_from_page(page), page);
- VM_BUG_ON_PAGE(hugetlb_cgroup_from_page_rsvd(page), page);
- set_compound_page_dtor(page, NULL_COMPOUND_DTOR);
- set_page_refcounted(page);
+
if (hstate_is_gigantic(h)) {
- /*
- * Temporarily drop the hugetlb_lock, because
- * we might block in free_gigantic_page().
- */
- spin_unlock(&hugetlb_lock);
destroy_compound_gigantic_page(page, huge_page_order(h));
free_gigantic_page(page, huge_page_order(h));
- spin_lock(&hugetlb_lock);
} else {
__free_pages(page, huge_page_order(h));
}
+
+ if (free_vmemmap_pages_per_hpage(h) || hstate_is_gigantic(h))
+ spin_lock(&hugetlb_lock);
+
+ return 0;
}

struct hstate *size_to_hstate(unsigned long size)
@@ -1404,9 +1443,9 @@ static void __free_huge_page(struct page *page)
} else if (h->surplus_huge_pages_node[nid]) {
/* remove the page from active list */
list_del(&page->lru);
- update_and_free_page(h, page);
h->surplus_huge_pages--;
h->surplus_huge_pages_node[nid]--;
+ update_and_free_page(h, page);
} else {
arch_clear_hugepage_flags(page);
enqueue_huge_page(h, page);
@@ -1447,7 +1486,7 @@ void free_huge_page(struct page *page)
/*
* Defer freeing if in non-task context to avoid hugetlb_lock deadlock.
*/
- if (!in_task()) {
+ if (!in_atomic()) {
/*
* Only call schedule_work() if hpage_freelist is previously
* empty. Otherwise, schedule_work() had been called but the
@@ -1699,8 +1738,7 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed,
h->surplus_huge_pages--;
h->surplus_huge_pages_node[node]--;
}
- update_and_free_page(h, page);
- ret = 1;
+ ret = !update_and_free_page(h, page);
break;
}
}
@@ -1713,10 +1751,14 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed,
* nothing for in-use hugepages and non-hugepages.
* This function returns values like below:
*
- * -EBUSY: failed to dissolved free hugepages or the hugepage is in-use
- * (allocated or reserved.)
- * 0: successfully dissolved free hugepages or the page is not a
- * hugepage (considered as already dissolved)
+ * -ENOMEM: failed to allocate vmemmap pages to free the freed hugepages
+ * when the system is under memory pressure and the feature of
+ * freeing unused vmemmap pages associated with each hugetlb page
+ * is enabled.
+ * -EBUSY: failed to dissolved free hugepages or the hugepage is in-use
+ * (allocated or reserved.)
+ * 0: successfully dissolved free hugepages or the page is not a
+ * hugepage (considered as already dissolved)
*/
int dissolve_free_huge_page(struct page *page)
{
@@ -1771,8 +1813,12 @@ int dissolve_free_huge_page(struct page *page)
h->free_huge_pages--;
h->free_huge_pages_node[nid]--;
h->max_huge_pages--;
- update_and_free_page(h, head);
- rc = 0;
+ rc = update_and_free_page(h, head);
+ if (rc) {
+ h->surplus_huge_pages--;
+ h->surplus_huge_pages_node[nid]--;
+ h->max_huge_pages++;
+ }
}
out:
spin_unlock(&hugetlb_lock);
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 0209b736e0b4..f7ab3d99250a 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -181,21 +181,31 @@
#define RESERVE_VMEMMAP_NR 2U
#define RESERVE_VMEMMAP_SIZE (RESERVE_VMEMMAP_NR << PAGE_SHIFT)

-/*
- * How many vmemmap pages associated with a HugeTLB page that can be freed
- * to the buddy allocator.
- *
- * Todo: Returns zero for now, which means the feature is disabled. We will
- * enable it once all the infrastructure is there.
- */
-static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
+static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h)
{
- return 0;
+ return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT;
}

-static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h)
+int alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
{
- return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT;
+ unsigned long vmemmap_addr = (unsigned long)head;
+ unsigned long vmemmap_end, vmemmap_reuse;
+
+ if (!free_vmemmap_pages_per_hpage(h))
+ return 0;
+
+ vmemmap_addr += RESERVE_VMEMMAP_SIZE;
+ vmemmap_end = vmemmap_addr + free_vmemmap_pages_size_per_hpage(h);
+ vmemmap_reuse = vmemmap_addr - PAGE_SIZE;
+ /*
+ * The pages which the vmemmap virtual address range [@vmemmap_addr,
+ * @vmemmap_end) are mapped to are freed to the buddy allocator, and
+ * the range is mapped to the page which @vmemmap_reuse is mapped to.
+ * When a HugeTLB page is freed to the buddy allocator, previously
+ * discarded vmemmap pages must be allocated and remapping.
+ */
+ return vmemmap_remap_alloc(vmemmap_addr, vmemmap_end, vmemmap_reuse,
+ GFP_KERNEL | __GFP_NORETRY | __GFP_THISNODE);
}

void free_huge_page_vmemmap(struct hstate *h, struct page *head)
diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
index 6923f03534d5..a37771b0b82a 100644
--- a/mm/hugetlb_vmemmap.h
+++ b/mm/hugetlb_vmemmap.h
@@ -11,10 +11,33 @@
#include <linux/hugetlb.h>

#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
+int alloc_huge_page_vmemmap(struct hstate *h, struct page *head);
void free_huge_page_vmemmap(struct hstate *h, struct page *head);
+
+/*
+ * How many vmemmap pages associated with a HugeTLB page that can be freed
+ * to the buddy allocator.
+ *
+ * Todo: Returns zero for now, which means the feature is disabled. We will
+ * enable it once all the infrastructure is there.
+ */
+static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
+{
+ return 0;
+}
#else
+static inline int alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
+{
+ return 0;
+}
+
static inline void free_huge_page_vmemmap(struct hstate *h, struct page *head)
{
}
+
+static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
+{
+ return 0;
+}
#endif /* CONFIG_HUGETLB_PAGE_FREE_VMEMMAP */
#endif /* _LINUX_HUGETLB_VMEMMAP_H */
diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
index d3076a7a3783..60fc6cd6cd23 100644
--- a/mm/sparse-vmemmap.c
+++ b/mm/sparse-vmemmap.c
@@ -40,7 +40,8 @@
* @remap_pte: called for each lowest-level entry (PTE).
* @reuse_page: the page which is reused for the tail vmemmap pages.
* @reuse_addr: the virtual address of the @reuse_page page.
- * @vmemmap_pages: the list head of the vmemmap pages that can be freed.
+ * @vmemmap_pages: the list head of the vmemmap pages that can be freed
+ * or is mapped from.
*/
struct vmemmap_remap_walk {
void (*remap_pte)(pte_t *pte, unsigned long addr,
@@ -237,6 +238,78 @@ void vmemmap_remap_free(unsigned long start, unsigned long end,
free_vmemmap_page_list(&vmemmap_pages);
}

+static void vmemmap_restore_pte(pte_t *pte, unsigned long addr,
+ struct vmemmap_remap_walk *walk)
+{
+ pgprot_t pgprot = PAGE_KERNEL;
+ struct page *page;
+ void *to;
+
+ BUG_ON(pte_page(*pte) != walk->reuse_page);
+
+ page = list_first_entry(walk->vmemmap_pages, struct page, lru);
+ list_del(&page->lru);
+ to = page_to_virt(page);
+ copy_page(to, (void *)walk->reuse_addr);
+
+ set_pte_at(&init_mm, addr, pte, mk_pte(page, pgprot));
+}
+
+static int alloc_vmemmap_page_list(unsigned long start, unsigned long end,
+ gfp_t gfp_mask, struct list_head *list)
+{
+ unsigned long nr_pages = (end - start) >> PAGE_SHIFT;
+ int nid = page_to_nid((struct page *)start);
+ struct page *page, *next;
+
+ while (nr_pages--) {
+ page = alloc_pages_node(nid, gfp_mask, 0);
+ if (!page)
+ goto out;
+ list_add_tail(&page->lru, list);
+ }
+
+ return 0;
+out:
+ list_for_each_entry_safe(page, next, list, lru)
+ __free_pages(page, 0);
+ return -ENOMEM;
+}
+
+/**
+ * vmemmap_remap_alloc - remap the vmemmap virtual address range [@start, end)
+ * to the page which is from the @vmemmap_pages
+ * respectively.
+ * @start: start address of the vmemmap virtual address range that we want
+ * to remap.
+ * @end: end address of the vmemmap virtual address range that we want to
+ * remap.
+ * @reuse: reuse address.
+ * @gpf_mask: GFP flag for allocating vmemmap pages.
+ */
+int vmemmap_remap_alloc(unsigned long start, unsigned long end,
+ unsigned long reuse, gfp_t gfp_mask)
+{
+ LIST_HEAD(vmemmap_pages);
+ struct vmemmap_remap_walk walk = {
+ .remap_pte = vmemmap_restore_pte,
+ .reuse_addr = reuse,
+ .vmemmap_pages = &vmemmap_pages,
+ };
+
+ /* See the comment in the vmemmap_remap_free(). */
+ BUG_ON(start - reuse != PAGE_SIZE);
+
+ might_sleep_if(gfpflags_allow_blocking(gfp_mask));
+
+ if (alloc_vmemmap_page_list(start, end, gfp_mask, &vmemmap_pages))
+ return -ENOMEM;
+
+ vmemmap_remap_range(reuse, end, &walk);
+
+ return 0;
+}
+
/*
* Allocate a block of memory to be used to back the virtual memory map
* or to back the page tables that are used to create the mapping.
--
2.11.0

2021-02-25 13:29:16

by Muchun Song

Subject: [PATCH v17 3/9] mm: hugetlb: free the vmemmap pages associated with each HugeTLB page

Every HugeTLB has more than one struct page structure. We __know__ that
we only use the first 4(HUGETLB_CGROUP_MIN_ORDER) struct page structures
to store metadata associated with each HugeTLB.

There are a lot of struct page structures associated with each HugeTLB
page. For tail pages, the value of compound_head is the same. So we can
reuse first page of tail page structures. We map the virtual addresses
of the remaining pages of tail page structures to the first tail page
struct, and then free these page frames. Therefore, we need to reserve
two pages as vmemmap areas.

When we allocate a HugeTLB page from the buddy, we can free some vmemmap
pages associated with each HugeTLB page. It is more appropriate to do it
in the prep_new_huge_page().

The free_vmemmap_pages_per_hpage(), which indicates how many vmemmap
pages associated with a HugeTLB page can be freed, returns zero for
now, which means the feature is disabled. We will enable it once all
the infrastructure is there.

Signed-off-by: Muchun Song <[email protected]>
Reviewed-by: Oscar Salvador <[email protected]>
---
include/linux/bootmem_info.h | 27 +++++-
include/linux/mm.h | 3 +
mm/Makefile | 1 +
mm/hugetlb.c | 3 +
mm/hugetlb_vmemmap.c | 219 +++++++++++++++++++++++++++++++++++++++++++
mm/hugetlb_vmemmap.h | 20 ++++
mm/sparse-vmemmap.c | 207 ++++++++++++++++++++++++++++++++++++++++
7 files changed, 479 insertions(+), 1 deletion(-)
create mode 100644 mm/hugetlb_vmemmap.c
create mode 100644 mm/hugetlb_vmemmap.h

diff --git a/include/linux/bootmem_info.h b/include/linux/bootmem_info.h
index 4ed6dee1adc9..ec03a624dfa2 100644
--- a/include/linux/bootmem_info.h
+++ b/include/linux/bootmem_info.h
@@ -2,7 +2,7 @@
#ifndef __LINUX_BOOTMEM_INFO_H
#define __LINUX_BOOTMEM_INFO_H

-#include <linux/mmzone.h>
+#include <linux/mm.h>

/*
* Types for free bootmem stored in page->lru.next. These have to be in
@@ -22,6 +22,27 @@ void __init register_page_bootmem_info_node(struct pglist_data *pgdat);
void get_page_bootmem(unsigned long info, struct page *page,
unsigned long type);
void put_page_bootmem(struct page *page);
+
+/*
+ * Any memory allocated via the memblock allocator and not via the
+ * buddy will be marked reserved already in the memmap. For those
+ * pages, we can call this function to free it to buddy allocator.
+ */
+static inline void free_bootmem_page(struct page *page)
+{
+ unsigned long magic = (unsigned long)page->freelist;
+
+ /*
+ * The reserve_bootmem_region sets the reserved flag on bootmem
+ * pages.
+ */
+ VM_BUG_ON_PAGE(page_ref_count(page) != 2, page);
+
+ if (magic == SECTION_INFO || magic == MIX_SECTION_INFO)
+ put_page_bootmem(page);
+ else
+ VM_BUG_ON_PAGE(1, page);
+}
#else
static inline void register_page_bootmem_info_node(struct pglist_data *pgdat)
{
@@ -35,6 +56,10 @@ static inline void get_page_bootmem(unsigned long info, struct page *page,
unsigned long type)
{
}
+
+static inline void free_bootmem_page(struct page *page)
+{
+}
#endif

#endif /* __LINUX_BOOTMEM_INFO_H */
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 77e64e3eac80..4ddfc31f21c6 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2971,6 +2971,9 @@ static inline void print_vma_addr(char *prefix, unsigned long rip)
}
#endif

+void vmemmap_remap_free(unsigned long start, unsigned long end,
+ unsigned long reuse);
+
void *sparse_buffer_alloc(unsigned long size);
struct page * __populate_section_memmap(unsigned long pfn,
unsigned long nr_pages, int nid, struct vmem_altmap *altmap);
diff --git a/mm/Makefile b/mm/Makefile
index daabf86d7da8..3d7d57e3b55b 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -71,6 +71,7 @@ obj-$(CONFIG_FRONTSWAP) += frontswap.o
obj-$(CONFIG_ZSWAP) += zswap.o
obj-$(CONFIG_HAS_DMA) += dmapool.o
obj-$(CONFIG_HUGETLBFS) += hugetlb.o
+obj-$(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP) += hugetlb_vmemmap.o
obj-$(CONFIG_NUMA) += mempolicy.o
obj-$(CONFIG_SPARSEMEM) += sparse.o
obj-$(CONFIG_SPARSEMEM_VMEMMAP) += sparse-vmemmap.o
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index c232cb67dda2..43fed6785322 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -42,6 +42,7 @@
#include <linux/userfaultfd_k.h>
#include <linux/page_owner.h>
#include "internal.h"
+#include "hugetlb_vmemmap.h"

int hugetlb_max_hstate __read_mostly;
unsigned int default_hstate_idx;
@@ -1463,6 +1464,8 @@ void free_huge_page(struct page *page)

static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
{
+ free_huge_page_vmemmap(h, page);
+
INIT_LIST_HEAD(&page->lru);
set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
set_hugetlb_cgroup(page, NULL);
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
new file mode 100644
index 000000000000..0209b736e0b4
--- /dev/null
+++ b/mm/hugetlb_vmemmap.c
@@ -0,0 +1,219 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Free some vmemmap pages of HugeTLB
+ *
+ * Copyright (c) 2020, Bytedance. All rights reserved.
+ *
+ * Author: Muchun Song <[email protected]>
+ *
+ * The struct page structures (page structs) are used to describe a physical
+ * page frame. By default, there is a one-to-one mapping from a page frame to
+ * it's corresponding page struct.
+ *
+ * HugeTLB pages consist of multiple base page size pages and is supported by
+ * many architectures. See hugetlbpage.rst in the Documentation directory for
+ * more details. On the x86-64 architecture, HugeTLB pages of size 2MB and 1GB
+ * are currently supported. Since the base page size on x86 is 4KB, a 2MB
+ * HugeTLB page consists of 512 base pages and a 1GB HugeTLB page consists of
+ * 4096 base pages. For each base page, there is a corresponding page struct.
+ *
+ * Within the HugeTLB subsystem, only the first 4 page structs are used to
+ * contain unique information about a HugeTLB page. HUGETLB_CGROUP_MIN_ORDER
+ * provides this upper limit. The only 'useful' information in the remaining
+ * page structs is the compound_head field, and this field is the same for all
+ * tail pages.
+ *
+ * By removing redundant page structs for HugeTLB pages, memory can be returned
+ * to the buddy allocator for other uses.
+ *
+ * Different architectures support different HugeTLB pages. For example, the
+ * following table is the HugeTLB page size supported by x86 and arm64
+ * architectures. Because arm64 supports 4k, 16k, and 64k base pages and
+ * supports contiguous entries, so it supports many kinds of sizes of HugeTLB
+ * page.
+ *
+ * +--------------+-----------+-----------------------------------------------+
+ * | Architecture | Page Size | HugeTLB Page Size |
+ * +--------------+-----------+-----------+-----------+-----------+-----------+
+ * | x86-64 | 4KB | 2MB | 1GB | | |
+ * +--------------+-----------+-----------+-----------+-----------+-----------+
+ * | | 4KB | 64KB | 2MB | 32MB | 1GB |
+ * | +-----------+-----------+-----------+-----------+-----------+
+ * | arm64 | 16KB | 2MB | 32MB | 1GB | |
+ * | +-----------+-----------+-----------+-----------+-----------+
+ * | | 64KB | 2MB | 512MB | 16GB | |
+ * +--------------+-----------+-----------+-----------+-----------+-----------+
+ *
+ * When the system boot up, every HugeTLB page has more than one struct page
+ * structs which size is (unit: pages):
+ *
+ * struct_size = HugeTLB_Size / PAGE_SIZE * sizeof(struct page) / PAGE_SIZE
+ *
+ * Where HugeTLB_Size is the size of the HugeTLB page. We know that the size
+ * of the HugeTLB page is always n times PAGE_SIZE. So we can get the following
+ * relationship.
+ *
+ * HugeTLB_Size = n * PAGE_SIZE
+ *
+ * Then,
+ *
+ * struct_size = n * PAGE_SIZE / PAGE_SIZE * sizeof(struct page) / PAGE_SIZE
+ * = n * sizeof(struct page) / PAGE_SIZE
+ *
+ * We can use huge mapping at the pud/pmd level for the HugeTLB page.
+ *
+ * For the HugeTLB page of the pmd level mapping, then
+ *
+ * struct_size = n * sizeof(struct page) / PAGE_SIZE
+ * = PAGE_SIZE / sizeof(pte_t) * sizeof(struct page) / PAGE_SIZE
+ * = sizeof(struct page) / sizeof(pte_t)
+ * = 64 / 8
+ * = 8 (pages)
+ *
+ * Where n is how many pte entries which one page can contains. So the value of
+ * n is (PAGE_SIZE / sizeof(pte_t)).
+ *
+ * This optimization only supports 64-bit system, so the value of sizeof(pte_t)
+ * is 8. And this optimization also applicable only when the size of struct page
+ * is a power of two. In most cases, the size of struct page is 64 bytes (e.g.
+ * x86-64 and arm64). So if we use pmd level mapping for a HugeTLB page, the
+ * size of struct page structs of it is 8 page frames which size depends on the
+ * size of the base page.
+ *
+ * For the HugeTLB page of the pud level mapping, then
+ *
+ * struct_size = PAGE_SIZE / sizeof(pmd_t) * struct_size(pmd)
+ * = PAGE_SIZE / 8 * 8 (pages)
+ * = PAGE_SIZE (pages)
+ *
+ * Where the struct_size(pmd) is the size of the struct page structs of a
+ * HugeTLB page of the pmd level mapping.
+ *
+ * E.g.: A 2MB HugeTLB page on x86_64 consists in 8 page frames while 1GB
+ * HugeTLB page consists in 4096.
+ *
+ * Next, we take the pmd level mapping of the HugeTLB page as an example to
+ * show the internal implementation of this optimization. There are 8 pages
+ * struct page structs associated with a HugeTLB page which is pmd mapped.
+ *
+ * Here is how things look before optimization.
+ *
+ * HugeTLB struct pages(8 pages) page frame(8 pages)
+ * +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+
+ * | | | 0 | -------------> | 0 |
+ * | | +-----------+ +-----------+
+ * | | | 1 | -------------> | 1 |
+ * | | +-----------+ +-----------+
+ * | | | 2 | -------------> | 2 |
+ * | | +-----------+ +-----------+
+ * | | | 3 | -------------> | 3 |
+ * | | +-----------+ +-----------+
+ * | | | 4 | -------------> | 4 |
+ * | PMD | +-----------+ +-----------+
+ * | level | | 5 | -------------> | 5 |
+ * | mapping | +-----------+ +-----------+
+ * | | | 6 | -------------> | 6 |
+ * | | +-----------+ +-----------+
+ * | | | 7 | -------------> | 7 |
+ * | | +-----------+ +-----------+
+ * | |
+ * | |
+ * | |
+ * +-----------+
+ *
+ * The value of page->compound_head is the same for all tail pages. The first
+ * page of page structs (page 0) associated with the HugeTLB page contains the 4
+ * page structs necessary to describe the HugeTLB. The only use of the remaining
+ * pages of page structs (page 1 to page 7) is to point to page->compound_head.
+ * Therefore, we can remap pages 2 to 7 to page 1. Only 2 pages of page structs
+ * will be used for each HugeTLB page. This will allow us to free the remaining
+ * 6 pages to the buddy allocator.
+ *
+ * Here is how things look after remapping.
+ *
+ * HugeTLB struct pages(8 pages) page frame(8 pages)
+ * +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+
+ * | | | 0 | -------------> | 0 |
+ * | | +-----------+ +-----------+
+ * | | | 1 | -------------> | 1 |
+ * | | +-----------+ +-----------+
+ * | | | 2 | ----------------^ ^ ^ ^ ^ ^
+ * | | +-----------+ | | | | |
+ * | | | 3 | ------------------+ | | | |
+ * | | +-----------+ | | | |
+ * | | | 4 | --------------------+ | | |
+ * | PMD | +-----------+ | | |
+ * | level | | 5 | ----------------------+ | |
+ * | mapping | +-----------+ | |
+ * | | | 6 | ------------------------+ |
+ * | | +-----------+ |
+ * | | | 7 | --------------------------+
+ * | | +-----------+
+ * | |
+ * | |
+ * | |
+ * +-----------+
+ *
+ * When a HugeTLB is freed to the buddy system, we should allocate 6 pages for
+ * vmemmap pages and restore the previous mapping relationship.
+ *
+ * For the HugeTLB page of the pud level mapping. It is similar to the former.
+ * We also can use this approach to free (PAGE_SIZE - 2) vmemmap pages.
+ *
+ * Apart from the HugeTLB page of the pmd/pud level mapping, some architectures
+ * (e.g. aarch64) provides a contiguous bit in the translation table entries
+ * that hints to the MMU to indicate that it is one of a contiguous set of
+ * entries that can be cached in a single TLB entry.
+ *
+ * The contiguous bit is used to increase the mapping size at the pmd and pte
+ * (last) level. So this type of HugeTLB page can be optimized only when its
+ * size of the struct page structs is greater than 2 pages.
+ */
+#include "hugetlb_vmemmap.h"
+
+/*
+ * There are a lot of struct page structures associated with each HugeTLB page.
+ * For tail pages, the value of compound_head is the same. So we can reuse first
+ * page of tail page structures. We map the virtual addresses of the remaining
+ * pages of tail page structures to the first tail page struct, and then free
+ * these page frames. Therefore, we need to reserve two pages as vmemmap areas.
+ */
+#define RESERVE_VMEMMAP_NR 2U
+#define RESERVE_VMEMMAP_SIZE (RESERVE_VMEMMAP_NR << PAGE_SHIFT)
+
+/*
+ * How many vmemmap pages associated with a HugeTLB page that can be freed
+ * to the buddy allocator.
+ *
+ * Todo: Returns zero for now, which means the feature is disabled. We will
+ * enable it once all the infrastructure is there.
+ */
+static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
+{
+ return 0;
+}
+
+static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h)
+{
+ return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT;
+}
+
+void free_huge_page_vmemmap(struct hstate *h, struct page *head)
+{
+ unsigned long vmemmap_addr = (unsigned long)head;
+ unsigned long vmemmap_end, vmemmap_reuse;
+
+ if (!free_vmemmap_pages_per_hpage(h))
+ return;
+
+ vmemmap_addr += RESERVE_VMEMMAP_SIZE;
+ vmemmap_end = vmemmap_addr + free_vmemmap_pages_size_per_hpage(h);
+ vmemmap_reuse = vmemmap_addr - PAGE_SIZE;
+
+ /*
+ * Remap the vmemmap virtual address range [@vmemmap_addr, @vmemmap_end)
+ * to the page which @vmemmap_reuse is mapped to, then free the pages
+ * which the range [@vmemmap_addr, @vmemmap_end] is mapped to.
+ */
+ vmemmap_remap_free(vmemmap_addr, vmemmap_end, vmemmap_reuse);
+}
diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
new file mode 100644
index 000000000000..6923f03534d5
--- /dev/null
+++ b/mm/hugetlb_vmemmap.h
@@ -0,0 +1,20 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Free some vmemmap pages of HugeTLB
+ *
+ * Copyright (c) 2020, Bytedance. All rights reserved.
+ *
+ * Author: Muchun Song <[email protected]>
+ */
+#ifndef _LINUX_HUGETLB_VMEMMAP_H
+#define _LINUX_HUGETLB_VMEMMAP_H
+#include <linux/hugetlb.h>
+
+#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
+void free_huge_page_vmemmap(struct hstate *h, struct page *head);
+#else
+static inline void free_huge_page_vmemmap(struct hstate *h, struct page *head)
+{
+}
+#endif /* CONFIG_HUGETLB_PAGE_FREE_VMEMMAP */
+#endif /* _LINUX_HUGETLB_VMEMMAP_H */
diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
index 16183d85a7d5..d3076a7a3783 100644
--- a/mm/sparse-vmemmap.c
+++ b/mm/sparse-vmemmap.c
@@ -27,8 +27,215 @@
#include <linux/spinlock.h>
#include <linux/vmalloc.h>
#include <linux/sched.h>
+#include <linux/pgtable.h>
+#include <linux/bootmem_info.h>
+
#include <asm/dma.h>
#include <asm/pgalloc.h>
+#include <asm/tlbflush.h>
+
+/**
+ * vmemmap_remap_walk - walk vmemmap page table
+ *
+ * @remap_pte: called for each lowest-level entry (PTE).
+ * @reuse_page: the page which is reused for the tail vmemmap pages.
+ * @reuse_addr: the virtual address of the @reuse_page page.
+ * @vmemmap_pages: the list head of the vmemmap pages that can be freed.
+ */
+struct vmemmap_remap_walk {
+ void (*remap_pte)(pte_t *pte, unsigned long addr,
+ struct vmemmap_remap_walk *walk);
+ struct page *reuse_page;
+ unsigned long reuse_addr;
+ struct list_head *vmemmap_pages;
+};
+
+static void vmemmap_pte_range(pmd_t *pmd, unsigned long addr,
+ unsigned long end,
+ struct vmemmap_remap_walk *walk)
+{
+ pte_t *pte;
+
+ pte = pte_offset_kernel(pmd, addr);
+
+ /*
+ * The reuse_page is found 'first' in table walk before we start
+ * remapping (which is calling @walk->remap_pte).
+ */
+ if (!walk->reuse_page) {
+ BUG_ON(pte_none(*pte));
+ BUG_ON(walk->reuse_addr != addr);
+
+ walk->reuse_page = pte_page(*pte++);
+ /*
+ * Because the reuse address is part of the range that we are
+ * walking, skip the reuse address range.
+ */
+ addr += PAGE_SIZE;
+ }
+
+ for (; addr != end; addr += PAGE_SIZE, pte++) {
+ BUG_ON(pte_none(*pte));
+
+ walk->remap_pte(pte, addr, walk);
+ }
+}
+
+static void vmemmap_pmd_range(pud_t *pud, unsigned long addr,
+ unsigned long end,
+ struct vmemmap_remap_walk *walk)
+{
+ pmd_t *pmd;
+ unsigned long next;
+
+ pmd = pmd_offset(pud, addr);
+ do {
+ BUG_ON(pmd_none(*pmd) || pmd_leaf(*pmd));
+
+ next = pmd_addr_end(addr, end);
+ vmemmap_pte_range(pmd, addr, next, walk);
+ } while (pmd++, addr = next, addr != end);
+}
+
+static void vmemmap_pud_range(p4d_t *p4d, unsigned long addr,
+ unsigned long end,
+ struct vmemmap_remap_walk *walk)
+{
+ pud_t *pud;
+ unsigned long next;
+
+ pud = pud_offset(p4d, addr);
+ do {
+ BUG_ON(pud_none(*pud));
+
+ next = pud_addr_end(addr, end);
+ vmemmap_pmd_range(pud, addr, next, walk);
+ } while (pud++, addr = next, addr != end);
+}
+
+static void vmemmap_p4d_range(pgd_t *pgd, unsigned long addr,
+ unsigned long end,
+ struct vmemmap_remap_walk *walk)
+{
+ p4d_t *p4d;
+ unsigned long next;
+
+ p4d = p4d_offset(pgd, addr);
+ do {
+ BUG_ON(p4d_none(*p4d));
+
+ next = p4d_addr_end(addr, end);
+ vmemmap_pud_range(p4d, addr, next, walk);
+ } while (p4d++, addr = next, addr != end);
+}
+
+static void vmemmap_remap_range(unsigned long start, unsigned long end,
+ struct vmemmap_remap_walk *walk)
+{
+ unsigned long addr = start;
+ unsigned long next;
+ pgd_t *pgd;
+
+ VM_BUG_ON(!IS_ALIGNED(start, PAGE_SIZE));
+ VM_BUG_ON(!IS_ALIGNED(end, PAGE_SIZE));
+
+ pgd = pgd_offset_k(addr);
+ do {
+ BUG_ON(pgd_none(*pgd));
+
+ next = pgd_addr_end(addr, end);
+ vmemmap_p4d_range(pgd, addr, next, walk);
+ } while (pgd++, addr = next, addr != end);
+
+ /*
+ * We only change the mapping of the vmemmap virtual address range
+ * [@start + PAGE_SIZE, end), so we only need to flush the TLB which
+ * belongs to the range.
+ */
+ flush_tlb_kernel_range(start + PAGE_SIZE, end);
+}
+
+/*
+ * Free a vmemmap page. A vmemmap page can be allocated from the memblock
+ * allocator or buddy allocator. If the PG_reserved flag is set, it means
+ * that it allocated from the memblock allocator, just free it via the
+ * free_bootmem_page(). Otherwise, use __free_page().
+ */
+static inline void free_vmemmap_page(struct page *page)
+{
+ if (PageReserved(page))
+ free_bootmem_page(page);
+ else
+ __free_page(page);
+}
+
+/* Free a list of the vmemmap pages */
+static void free_vmemmap_page_list(struct list_head *list)
+{
+ struct page *page, *next;
+
+ list_for_each_entry_safe(page, next, list, lru) {
+ list_del(&page->lru);
+ free_vmemmap_page(page);
+ }
+}
+
+static void vmemmap_remap_pte(pte_t *pte, unsigned long addr,
+ struct vmemmap_remap_walk *walk)
+{
+ /*
+ * Remap the tail pages as read-only to catch illegal write operation
+ * to the tail pages.
+ */
+ pgprot_t pgprot = PAGE_KERNEL_RO;
+ pte_t entry = mk_pte(walk->reuse_page, pgprot);
+ struct page *page = pte_page(*pte);
+
+ list_add(&page->lru, walk->vmemmap_pages);
+ set_pte_at(&init_mm, addr, pte, entry);
+}
+
+/**
+ * vmemmap_remap_free - remap the vmemmap virtual address range [@start, @end)
+ * to the page which @reuse is mapped to, then free vmemmap
+ * which the range are mapped to.
+ * @start: start address of the vmemmap virtual address range that we want
+ * to remap.
+ * @end: end address of the vmemmap virtual address range that we want to
+ * remap.
+ * @reuse: reuse address.
+ *
+ * Note: This function depends on vmemmap being base page mapped. Please make
+ * sure that we disable PMD mapping of vmemmap pages when calling this function.
+ */
+void vmemmap_remap_free(unsigned long start, unsigned long end,
+ unsigned long reuse)
+{
+ LIST_HEAD(vmemmap_pages);
+ struct vmemmap_remap_walk walk = {
+ .remap_pte = vmemmap_remap_pte,
+ .reuse_addr = reuse,
+ .vmemmap_pages = &vmemmap_pages,
+ };
+
+ /*
+ * In order to make remapping routine most efficient for the huge pages,
+ * the routine of vmemmap page table walking has the following rules
+ * (see more details from the vmemmap_pte_range()):
+ *
+ * - The range [@start, @end) and the range [@reuse, @reuse + PAGE_SIZE)
+ * should be continuous.
+ * - The @reuse address is part of the range [@reuse, @end) that we are
+ * walking which is passed to vmemmap_remap_range().
+ * - The @reuse address is the first in the complete range.
+ *
+ * So we need to make sure that @start and @reuse meet the above rules.
+ */
+ BUG_ON(start - reuse != PAGE_SIZE);
+
+ vmemmap_remap_range(reuse, end, &walk);
+ free_vmemmap_page_list(&vmemmap_pages);
+}

/*
* Allocate a block of memory to be used to back the virtual memory map
--
2.11.0

2021-02-25 13:29:57

by Muchun Song

Subject: [PATCH v17 8/9] mm: hugetlb: gather discrete indexes of tail page

For HugeTLB page, there are more metadata to save in the struct page.
But the head struct page cannot meet our needs, so we have to abuse
other tail struct page to store the metadata. In order to avoid
conflicts caused by subsequent use of more tail struct pages, we can
gather these discrete indexes of tail struct page. In this case, it
will be easier to add a new tail page index later.

There are only (RESERVE_VMEMMAP_SIZE / sizeof(struct page)) struct
page structs that can be used when CONFIG_HUGETLB_PAGE_FREE_VMEMMAP,
so add a BUILD_BUG_ON to catch invalid usage of the tail struct page.

Signed-off-by: Muchun Song <[email protected]>
Reviewed-by: Oscar Salvador <[email protected]>
Reviewed-by: Miaohe Lin <[email protected]>
---
include/linux/hugetlb.h | 24 ++++++++++++++++++++++--
include/linux/hugetlb_cgroup.h | 19 +++++++++++--------
mm/hugetlb.c | 6 +++---
mm/hugetlb_vmemmap.c | 8 ++++++++
4 files changed, 44 insertions(+), 13 deletions(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index a4d80f7263fc..c70421e26189 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -28,6 +28,26 @@ typedef struct { unsigned long pd; } hugepd_t;
#include <linux/shm.h>
#include <asm/tlbflush.h>

+/*
+ * For HugeTLB page, there are more metadata to save in the struct page. But
+ * the head struct page cannot meet our needs, so we have to abuse other tail
+ * struct page to store the metadata. In order to avoid conflicts caused by
+ * subsequent use of more tail struct pages, we gather these discrete indexes
+ * of tail struct page here.
+ */
+enum {
+ SUBPAGE_INDEX_SUBPOOL = 1, /* reuse page->private */
+#ifdef CONFIG_CGROUP_HUGETLB
+ SUBPAGE_INDEX_CGROUP, /* reuse page->private */
+ SUBPAGE_INDEX_CGROUP_RSVD, /* reuse page->private */
+ __MAX_CGROUP_SUBPAGE_INDEX = SUBPAGE_INDEX_CGROUP_RSVD,
+#endif
+#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
+ SUBPAGE_INDEX_HWPOISON, /* reuse page->private */
+#endif
+ __NR_USED_SUBPAGE,
+};
+
struct hugepage_subpool {
spinlock_t lock;
long count;
@@ -607,13 +627,13 @@ extern unsigned int default_hstate_idx;
*/
static inline struct hugepage_subpool *hugetlb_page_subpool(struct page *hpage)
{
- return (struct hugepage_subpool *)(hpage+1)->private;
+ return (void *)page_private(hpage + SUBPAGE_INDEX_SUBPOOL);
}

static inline void hugetlb_set_page_subpool(struct page *hpage,
struct hugepage_subpool *subpool)
{
- set_page_private(hpage+1, (unsigned long)subpool);
+ set_page_private(hpage + SUBPAGE_INDEX_SUBPOOL, (unsigned long)subpool);
}

static inline struct hstate *hstate_file(struct file *f)
diff --git a/include/linux/hugetlb_cgroup.h b/include/linux/hugetlb_cgroup.h
index 2ad6e92f124a..54ec689e3c9c 100644
--- a/include/linux/hugetlb_cgroup.h
+++ b/include/linux/hugetlb_cgroup.h
@@ -21,15 +21,16 @@ struct hugetlb_cgroup;
struct resv_map;
struct file_region;

+#ifdef CONFIG_CGROUP_HUGETLB
/*
* Minimum page order trackable by hugetlb cgroup.
* At least 4 pages are necessary for all the tracking information.
- * The second tail page (hpage[2]) is the fault usage cgroup.
- * The third tail page (hpage[3]) is the reservation usage cgroup.
+ * The second tail page (hpage[SUBPAGE_INDEX_CGROUP]) is the fault
+ * usage cgroup. The third tail page (hpage[SUBPAGE_INDEX_CGROUP_RSVD])
+ * is the reservation usage cgroup.
*/
-#define HUGETLB_CGROUP_MIN_ORDER 2
+#define HUGETLB_CGROUP_MIN_ORDER order_base_2(__MAX_CGROUP_SUBPAGE_INDEX + 1)

-#ifdef CONFIG_CGROUP_HUGETLB
enum hugetlb_memory_event {
HUGETLB_MAX,
HUGETLB_NR_MEMORY_EVENTS,
@@ -66,9 +67,9 @@ __hugetlb_cgroup_from_page(struct page *page, bool rsvd)
if (compound_order(page) < HUGETLB_CGROUP_MIN_ORDER)
return NULL;
if (rsvd)
- return (struct hugetlb_cgroup *)page[3].private;
+ return (void *)page_private(page + SUBPAGE_INDEX_CGROUP_RSVD);
else
- return (struct hugetlb_cgroup *)page[2].private;
+ return (void *)page_private(page + SUBPAGE_INDEX_CGROUP);
}

static inline struct hugetlb_cgroup *hugetlb_cgroup_from_page(struct page *page)
@@ -90,9 +91,11 @@ static inline int __set_hugetlb_cgroup(struct page *page,
if (compound_order(page) < HUGETLB_CGROUP_MIN_ORDER)
return -1;
if (rsvd)
- page[3].private = (unsigned long)h_cg;
+ set_page_private(page + SUBPAGE_INDEX_CGROUP_RSVD,
+ (unsigned long)h_cg);
else
- page[2].private = (unsigned long)h_cg;
+ set_page_private(page + SUBPAGE_INDEX_CGROUP,
+ (unsigned long)h_cg);
return 0;
}

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 4d192ba183f9..31518b39f18d 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1312,7 +1312,7 @@ static inline void hwpoison_subpage_deliver(struct hstate *h, struct page *head)
if (!PageHWPoison(head) || !free_vmemmap_pages_per_hpage(h))
return;

- page = head + page_private(head + 4);
+ page = head + page_private(head + SUBPAGE_INDEX_HWPOISON);

/*
* Move PageHWPoison flag from head page to the raw error page,
@@ -1331,7 +1331,7 @@ static inline void hwpoison_subpage_set(struct hstate *h, struct page *head,
return;

if (free_vmemmap_pages_per_hpage(h)) {
- set_page_private(head + 4, page - head);
+ set_page_private(head + SUBPAGE_INDEX_HWPOISON, page - head);
} else if (page != head) {
/*
* Move PageHWPoison flag from head page to the raw error page,
@@ -1347,7 +1347,7 @@ static inline void hwpoison_subpage_clear(struct hstate *h, struct page *head)
if (!PageHWPoison(head) || !free_vmemmap_pages_per_hpage(h))
return;

- set_page_private(head + 4, 0);
+ set_page_private(head + SUBPAGE_INDEX_HWPOISON, 0);
}
#else
static inline void hwpoison_subpage_deliver(struct hstate *h, struct page *head)
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index b65f0d5189bd..33e42678abe3 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -257,6 +257,14 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
unsigned int nr_pages = pages_per_huge_page(h);
unsigned int vmemmap_pages;

+ /*
+ * There are only (RESERVE_VMEMMAP_SIZE / sizeof(struct page)) struct
+ * page structs that can be used when CONFIG_HUGETLB_PAGE_FREE_VMEMMAP,
+ * so add a BUILD_BUG_ON to catch invalid usage of the tail struct page.
+ */
+ BUILD_BUG_ON(__NR_USED_SUBPAGE >=
+ RESERVE_VMEMMAP_SIZE / sizeof(struct page));
+
if (!hugetlb_free_vmemmap_enabled)
return;

--
2.11.0

2021-02-25 13:29:57

by Muchun Song

Subject: [PATCH v17 7/9] mm: hugetlb: introduce nr_free_vmemmap_pages in the struct hstate

All the infrastructure is ready, so we introduce nr_free_vmemmap_pages
field in the hstate to indicate how many vmemmap pages associated with
a HugeTLB page that can be freed to buddy allocator. And initialize it
in the hugetlb_vmemmap_init(). This patch is actual enablement of the
feature.

Signed-off-by: Muchun Song <[email protected]>
Acked-by: Mike Kravetz <[email protected]>
Reviewed-by: Oscar Salvador <[email protected]>
Reviewed-by: Miaohe Lin <[email protected]>
---
include/linux/hugetlb.h | 3 +++
mm/hugetlb.c | 1 +
mm/hugetlb_vmemmap.c | 25 +++++++++++++++++++++++++
mm/hugetlb_vmemmap.h | 10 ++++++----
4 files changed, 35 insertions(+), 4 deletions(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 78934e9aeab6..a4d80f7263fc 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -560,6 +560,9 @@ struct hstate {
unsigned int nr_huge_pages_node[MAX_NUMNODES];
unsigned int free_huge_pages_node[MAX_NUMNODES];
unsigned int surplus_huge_pages_node[MAX_NUMNODES];
+#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
+ unsigned int nr_free_vmemmap_pages;
+#endif
#ifdef CONFIG_CGROUP_HUGETLB
/* cgroup control files */
struct cftype cgroup_files_dfl[7];
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index bccb6907833f..4d192ba183f9 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -3312,6 +3312,7 @@ void __init hugetlb_add_hstate(unsigned int order)
h->next_nid_to_free = first_memory_node;
snprintf(h->name, HSTATE_NAME_LEN, "hugepages-%lukB",
huge_page_size(h)/1024);
+ hugetlb_vmemmap_init(h);

parsed_hstate = h;
}
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 7807ed6678e0..b65f0d5189bd 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -251,3 +251,28 @@ void free_huge_page_vmemmap(struct hstate *h, struct page *head)
*/
vmemmap_remap_free(vmemmap_addr, vmemmap_end, vmemmap_reuse);
}
+
+void __init hugetlb_vmemmap_init(struct hstate *h)
+{
+ unsigned int nr_pages = pages_per_huge_page(h);
+ unsigned int vmemmap_pages;
+
+ if (!hugetlb_free_vmemmap_enabled)
+ return;
+
+ vmemmap_pages = (nr_pages * sizeof(struct page)) >> PAGE_SHIFT;
+ /*
+ * The head page and the first tail page are not to be freed to buddy
+ * allocator, the other pages will map to the first tail page, so they
+ * can be freed.
+ *
+ * Could RESERVE_VMEMMAP_NR be greater than @vmemmap_pages? It is true
+ * on some architectures (e.g. aarch64). See Documentation/arm64/
+ * hugetlbpage.rst for more details.
+ */
+ if (likely(vmemmap_pages > RESERVE_VMEMMAP_NR))
+ h->nr_free_vmemmap_pages = vmemmap_pages - RESERVE_VMEMMAP_NR;
+
+ pr_info("can free %d vmemmap pages for %s\n", h->nr_free_vmemmap_pages,
+ h->name);
+}
diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
index a37771b0b82a..cb2bef8f9e73 100644
--- a/mm/hugetlb_vmemmap.h
+++ b/mm/hugetlb_vmemmap.h
@@ -13,17 +13,15 @@
#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
int alloc_huge_page_vmemmap(struct hstate *h, struct page *head);
void free_huge_page_vmemmap(struct hstate *h, struct page *head);
+void hugetlb_vmemmap_init(struct hstate *h);

/*
* How many vmemmap pages associated with a HugeTLB page that can be freed
* to the buddy allocator.
- *
- * Todo: Returns zero for now, which means the feature is disabled. We will
- * enable it once all the infrastructure is there.
*/
static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
{
- return 0;
+ return h->nr_free_vmemmap_pages;
}
#else
static inline int alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
@@ -35,6 +33,10 @@ static inline void free_huge_page_vmemmap(struct hstate *h, struct page *head)
{
}

+static inline void hugetlb_vmemmap_init(struct hstate *h)
+{
+}
+
static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
{
return 0;
--
2.11.0

2021-02-25 13:30:17

by Muchun Song

Subject: [PATCH v17 9/9] mm: hugetlb: optimize the code with the help of the compiler

When the "struct page size" crosses page boundaries we cannot
make use of this feature. Let free_vmemmap_pages_per_hpage()
return zero if that is the case, most of the functions can be
optimized away.

Signed-off-by: Muchun Song <[email protected]>
Reviewed-by: Miaohe Lin <[email protected]>
Reviewed-by: Oscar Salvador <[email protected]>
---
include/linux/hugetlb.h | 3 ++-
mm/hugetlb_vmemmap.c | 7 +++++++
mm/hugetlb_vmemmap.h | 6 ++++++
3 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index c70421e26189..333dd0479fc2 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -880,7 +880,8 @@ extern bool hugetlb_free_vmemmap_enabled;

static inline bool is_hugetlb_free_vmemmap_enabled(void)
{
- return hugetlb_free_vmemmap_enabled;
+ return hugetlb_free_vmemmap_enabled &&
+ is_power_of_2(sizeof(struct page));
}
#else
static inline bool is_hugetlb_free_vmemmap_enabled(void)
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 33e42678abe3..1ba1ef45c48c 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -265,6 +265,13 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
BUILD_BUG_ON(__NR_USED_SUBPAGE >=
RESERVE_VMEMMAP_SIZE / sizeof(struct page));

+ /*
+ * The compiler can help us to optimize this function to null
+ * when the size of the struct page is not power of 2.
+ */
+ if (!is_power_of_2(sizeof(struct page)))
+ return;
+
if (!hugetlb_free_vmemmap_enabled)
return;

diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
index cb2bef8f9e73..29aaaf7b741e 100644
--- a/mm/hugetlb_vmemmap.h
+++ b/mm/hugetlb_vmemmap.h
@@ -21,6 +21,12 @@ void hugetlb_vmemmap_init(struct hstate *h);
*/
static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
{
+ /*
+ * This check aims to let the compiler help us optimize the code as
+ * much as possible.
+ */
+ if (!is_power_of_2(sizeof(struct page)))
+ return 0;
return h->nr_free_vmemmap_pages;
}
#else
--
2.11.0

2021-02-25 13:30:42

by Muchun Song

Subject: [PATCH v17 6/9] mm: hugetlb: add a kernel parameter hugetlb_free_vmemmap

Add a kernel parameter hugetlb_free_vmemmap to enable the feature of
freeing unused vmemmap pages associated with each hugetlb page on boot.

We disables PMD mapping of vmemmap pages for x86-64 arch when this
feature is enabled. Because vmemmap_remap_free() depends on vmemmap
being base page mapped.

Signed-off-by: Muchun Song <[email protected]>
Reviewed-by: Oscar Salvador <[email protected]>
Reviewed-by: Barry Song <[email protected]>
Reviewed-by: Miaohe Lin <[email protected]>
---
Documentation/admin-guide/kernel-parameters.txt | 14 ++++++++++++++
Documentation/admin-guide/mm/hugetlbpage.rst | 3 +++
arch/x86/mm/init_64.c | 8 ++++++--
include/linux/hugetlb.h | 19 +++++++++++++++++++
mm/hugetlb_vmemmap.c | 24 ++++++++++++++++++++++++
5 files changed, 66 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 04545725f187..de91d54573c4 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1557,6 +1557,20 @@
Documentation/admin-guide/mm/hugetlbpage.rst.
Format: size[KMG]

+ hugetlb_free_vmemmap=
+ [KNL] When CONFIG_HUGETLB_PAGE_FREE_VMEMMAP is set,
+ this controls freeing unused vmemmap pages associated
+ with each HugeTLB page. When this option is enabled,
+ we disable PMD/huge page mapping of vmemmap pages which
+ increase page table pages. So if a user/sysadmin only
+ uses a small number of HugeTLB pages (as a percentage
+ of system memory), they could end up using more memory
+ with hugetlb_free_vmemmap on as opposed to off.
+ Format: { on | off (default) }
+
+ on: enable the feature
+ off: disable the feature
+
hung_task_panic=
[KNL] Should the hung task detector generate panics.
Format: 0 | 1
diff --git a/Documentation/admin-guide/mm/hugetlbpage.rst b/Documentation/admin-guide/mm/hugetlbpage.rst
index 6988895d09a8..8abaeb144e44 100644
--- a/Documentation/admin-guide/mm/hugetlbpage.rst
+++ b/Documentation/admin-guide/mm/hugetlbpage.rst
@@ -153,6 +153,9 @@ default_hugepagesz

will all result in 256 2M huge pages being allocated. Valid default
huge page size is architecture dependent.
+hugetlb_free_vmemmap
+ When CONFIG_HUGETLB_PAGE_FREE_VMEMMAP is set, this enables freeing
+ unused vmemmap pages associated with each HugeTLB page.

When multiple huge page sizes are supported, ``/proc/sys/vm/nr_hugepages``
indicates the current number of pre-allocated huge pages of the default size.
diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index 0435bee2e172..39f88c5faadc 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -34,6 +34,7 @@
#include <linux/gfp.h>
#include <linux/kcore.h>
#include <linux/bootmem_info.h>
+#include <linux/hugetlb.h>

#include <asm/processor.h>
#include <asm/bios_ebda.h>
@@ -1557,7 +1558,8 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
{
int err;

- if (end - start < PAGES_PER_SECTION * sizeof(struct page))
+ if ((is_hugetlb_free_vmemmap_enabled() && !altmap) ||
+ end - start < PAGES_PER_SECTION * sizeof(struct page))
err = vmemmap_populate_basepages(start, end, node, NULL);
else if (boot_cpu_has(X86_FEATURE_PSE))
err = vmemmap_populate_hugepages(start, end, node, altmap);
@@ -1585,6 +1587,8 @@ void register_page_bootmem_memmap(unsigned long section_nr,
pmd_t *pmd;
unsigned int nr_pmd_pages;
struct page *page;
+ bool base_mapping = !boot_cpu_has(X86_FEATURE_PSE) ||
+ is_hugetlb_free_vmemmap_enabled();

for (; addr < end; addr = next) {
pte_t *pte = NULL;
@@ -1610,7 +1614,7 @@ void register_page_bootmem_memmap(unsigned long section_nr,
}
get_page_bootmem(section_nr, pud_page(*pud), MIX_SECTION_INFO);

- if (!boot_cpu_has(X86_FEATURE_PSE)) {
+ if (base_mapping) {
next = (addr + PAGE_SIZE) & PAGE_MASK;
pmd = pmd_offset(pud, addr);
if (pmd_none(*pmd))
diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index ce6533584eb7..78934e9aeab6 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -852,6 +852,20 @@ static inline void huge_ptep_modify_prot_commit(struct vm_area_struct *vma,
}
#endif

+#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
+extern bool hugetlb_free_vmemmap_enabled;
+
+static inline bool is_hugetlb_free_vmemmap_enabled(void)
+{
+ return hugetlb_free_vmemmap_enabled;
+}
+#else
+static inline bool is_hugetlb_free_vmemmap_enabled(void)
+{
+ return false;
+}
+#endif
+
#else /* CONFIG_HUGETLB_PAGE */
struct hstate {};

@@ -1005,6 +1019,11 @@ static inline void set_huge_swap_pte_at(struct mm_struct *mm, unsigned long addr
pte_t *ptep, pte_t pte, unsigned long sz)
{
}
+
+static inline bool is_hugetlb_free_vmemmap_enabled(void)
+{
+ return false;
+}
#endif /* CONFIG_HUGETLB_PAGE */

static inline spinlock_t *huge_pte_lock(struct hstate *h,
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index f7ab3d99250a..7807ed6678e0 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -169,6 +169,8 @@
* (last) level. So this type of HugeTLB page can be optimized only when its
* size of the struct page structs is greater than 2 pages.
*/
+#define pr_fmt(fmt) "HugeTLB: " fmt
+
#include "hugetlb_vmemmap.h"

/*
@@ -181,6 +183,28 @@
#define RESERVE_VMEMMAP_NR 2U
#define RESERVE_VMEMMAP_SIZE (RESERVE_VMEMMAP_NR << PAGE_SHIFT)

+bool hugetlb_free_vmemmap_enabled;
+
+static int __init early_hugetlb_free_vmemmap_param(char *buf)
+{
+ /* We cannot optimize if a "struct page" crosses page boundaries. */
+ if ((!is_power_of_2(sizeof(struct page)))) {
+ pr_warn("cannot free vmemmap pages because \"struct page\" crosses page boundaries\n");
+ return 0;
+ }
+
+ if (!buf)
+ return -EINVAL;
+
+ if (!strcmp(buf, "on"))
+ hugetlb_free_vmemmap_enabled = true;
+ else if (strcmp(buf, "off"))
+ return -EINVAL;
+
+ return 0;
+}
+early_param("hugetlb_free_vmemmap", early_hugetlb_free_vmemmap_param);
+
static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h)
{
return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT;
--
2.11.0

2021-02-25 13:31:53

by Muchun Song

Subject: [PATCH v17 5/9] mm: hugetlb: set the PageHWPoison to the raw error page

Because we reuse the first tail vmemmap page frame and remap it
with read-only, we cannot set the PageHWPosion on some tail pages.
So we can use the head[4].private (There are at least 128 struct
page structures associated with the optimized HugeTLB page, so
using head[4].private is safe) to record the real error page index
and set the raw error page PageHWPoison later.

Signed-off-by: Muchun Song <[email protected]>
Reviewed-by: Oscar Salvador <[email protected]>
Acked-by: David Rientjes <[email protected]>
---
mm/hugetlb.c | 80 ++++++++++++++++++++++++++++++++++++++++++++++++++++++------
1 file changed, 72 insertions(+), 8 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index b6e4e3f31ad2..bccb6907833f 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1304,6 +1304,74 @@ static inline void destroy_compound_gigantic_page(struct page *page,
unsigned int order) { }
#endif

+#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
+static inline void hwpoison_subpage_deliver(struct hstate *h, struct page *head)
+{
+ struct page *page;
+
+ if (!PageHWPoison(head) || !free_vmemmap_pages_per_hpage(h))
+ return;
+
+ page = head + page_private(head + 4);
+
+ /*
+ * Move PageHWPoison flag from head page to the raw error page,
+ * which makes any subpages rather than the error page reusable.
+ */
+ if (page != head) {
+ SetPageHWPoison(page);
+ ClearPageHWPoison(head);
+ }
+}
+
+static inline void hwpoison_subpage_set(struct hstate *h, struct page *head,
+ struct page *page)
+{
+ if (!PageHWPoison(head))
+ return;
+
+ if (free_vmemmap_pages_per_hpage(h)) {
+ set_page_private(head + 4, page - head);
+ } else if (page != head) {
+ /*
+ * Move PageHWPoison flag from head page to the raw error page,
+ * which makes any subpages rather than the error page reusable.
+ */
+ SetPageHWPoison(page);
+ ClearPageHWPoison(head);
+ }
+}
+
+static inline void hwpoison_subpage_clear(struct hstate *h, struct page *head)
+{
+ if (!PageHWPoison(head) || !free_vmemmap_pages_per_hpage(h))
+ return;
+
+ set_page_private(head + 4, 0);
+}
+#else
+static inline void hwpoison_subpage_deliver(struct hstate *h, struct page *head)
+{
+}
+
+static inline void hwpoison_subpage_set(struct hstate *h, struct page *head,
+ struct page *page)
+{
+ if (PageHWPoison(head) && page != head) {
+ /*
+ * Move PageHWPoison flag from head page to the raw error page,
+ * which makes any subpages rather than the error page reusable.
+ */
+ SetPageHWPoison(page);
+ ClearPageHWPoison(head);
+ }
+}
+
+static inline void hwpoison_subpage_clear(struct hstate *h, struct page *head)
+{
+}
+#endif
+
static int update_and_free_page(struct hstate *h, struct page *page)
__releases(&hugetlb_lock) __acquires(&hugetlb_lock)
{
@@ -1357,6 +1425,8 @@ static int update_and_free_page(struct hstate *h, struct page *page)
return -ENOMEM;
}

+ hwpoison_subpage_deliver(h, page);
+
for (i = 0; i < pages_per_huge_page(h);
i++, subpage = mem_map_next(subpage, page, i)) {
subpage->flags &= ~(1 << PG_locked | 1 << PG_error |
@@ -1801,14 +1871,7 @@ int dissolve_free_huge_page(struct page *page)
goto retry;
}

- /*
- * Move PageHWPoison flag from head page to the raw error page,
- * which makes any subpages rather than the error page reusable.
- */
- if (PageHWPoison(head) && page != head) {
- SetPageHWPoison(page);
- ClearPageHWPoison(head);
- }
+ hwpoison_subpage_set(h, head, page);
list_del(&head->lru);
h->free_huge_pages--;
h->free_huge_pages_node[nid]--;
@@ -1818,6 +1881,7 @@ int dissolve_free_huge_page(struct page *page)
h->surplus_huge_pages--;
h->surplus_huge_pages_node[nid]--;
h->max_huge_pages++;
+ hwpoison_subpage_clear(h, head);
}
}
out:
--
2.11.0

2021-03-01 06:09:39

by Muchun Song

Subject: Re: [PATCH v17 4/9] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page

On Thu, Feb 25, 2021 at 9:24 PM Muchun Song <[email protected]> wrote:
>
> When we free a HugeTLB page to the buddy allocator, we should allocate
> the vmemmap pages associated with it. But we may cannot allocate vmemmap
> pages when the system is under memory pressure, in this case, we just
> refuse to free the HugeTLB page instead of looping forever trying to
> allocate the pages. This changes some behavior (list below) on some
> corner cases.
>
> 1) Failing to free a huge page triggered by the user (decrease nr_pages).
>
> Need try again later by the user.
>
> 2) Failing to free a surplus huge page when freed by the application.
>
> Try again later when freeing a huge page next time.
>
> 3) Failing to dissolve a free huge page on ZONE_MOVABLE via
> offline_pages().
>
> This is a bit unfortunate if we have plenty of ZONE_MOVABLE memory
> but are low on kernel memory. For example, migration of huge pages
> would still work, however, dissolving the free page does not work.
> This is a corner cases. When the system is that much under memory
> pressure, offlining/unplug can be expected to fail. This is
> unfortunate because it prevents from the memory offlining which
> shouldn't happen for movable zones. People depending on the memory
> hotplug and movable zone should carefuly consider whether savings
> on unmovable memory are worth losing their hotplug functionality
> in some situations.
>
> 4) Failing to dissolve a huge page on CMA/ZONE_MOVABLE via
> alloc_contig_range() - once we have that handling in place. Mainly
> affects CMA and virtio-mem.
>
> Similar to 3). virito-mem will handle migration errors gracefully.
> CMA might be able to fallback on other free areas within the CMA
> region.
>
> Vmemmap pages are allocated from the page freeing context. In order for
> those allocations to be not disruptive (e.g. trigger oom killer)
> __GFP_NORETRY is used. hugetlb_lock is dropped for the allocation
> because a non sleeping allocation would be too fragile and it could fail
> too easily under memory pressure. GFP_ATOMIC or other modes to access
> memory reserves is not used because we want to prevent consuming
> reserves under heavy hugetlb freeing.

Hi,

Since this patch is the only patch that has no reviewed-by tag.
I hope someone (e.g. Mike, Oscar, David or Michal) could review
this. Thanks a lot.

>
> Signed-off-by: Muchun Song <[email protected]>
> ---
> Documentation/admin-guide/mm/hugetlbpage.rst | 8 +++
> include/linux/mm.h | 2 +
> mm/hugetlb.c | 92 +++++++++++++++++++++-------
> mm/hugetlb_vmemmap.c | 32 ++++++----
> mm/hugetlb_vmemmap.h | 23 +++++++
> mm/sparse-vmemmap.c | 75 ++++++++++++++++++++++-
> 6 files changed, 197 insertions(+), 35 deletions(-)
>
> diff --git a/Documentation/admin-guide/mm/hugetlbpage.rst b/Documentation/admin-guide/mm/hugetlbpage.rst
> index f7b1c7462991..6988895d09a8 100644
> --- a/Documentation/admin-guide/mm/hugetlbpage.rst
> +++ b/Documentation/admin-guide/mm/hugetlbpage.rst
> @@ -60,6 +60,10 @@ HugePages_Surp
> the pool above the value in ``/proc/sys/vm/nr_hugepages``. The
> maximum number of surplus huge pages is controlled by
> ``/proc/sys/vm/nr_overcommit_hugepages``.
> + Note: When the feature of freeing unused vmemmap pages associated
> + with each hugetlb page is enabled, the number of surplus huge pages
> + may be temporarily larger than the maximum number of surplus huge
> + pages when the system is under memory pressure.
> Hugepagesize
> is the default hugepage size (in Kb).
> Hugetlb
> @@ -80,6 +84,10 @@ returned to the huge page pool when freed by a task. A user with root
> privileges can dynamically allocate more or free some persistent huge pages
> by increasing or decreasing the value of ``nr_hugepages``.
>
> +Note: When the feature of freeing unused vmemmap pages associated with each
> +hugetlb page is enabled, we can fail to free the huge pages triggered by
> +the user when ths system is under memory pressure. Please try again later.
> +
> Pages that are used as huge pages are reserved inside the kernel and cannot
> be used for other purposes. Huge pages cannot be swapped out under
> memory pressure.
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 4ddfc31f21c6..77693c944a36 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2973,6 +2973,8 @@ static inline void print_vma_addr(char *prefix, unsigned long rip)
>
> void vmemmap_remap_free(unsigned long start, unsigned long end,
> unsigned long reuse);
> +int vmemmap_remap_alloc(unsigned long start, unsigned long end,
> + unsigned long reuse, gfp_t gfp_mask);
>
> void *sparse_buffer_alloc(unsigned long size);
> struct page * __populate_section_memmap(unsigned long pfn,
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 43fed6785322..b6e4e3f31ad2 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1304,16 +1304,59 @@ static inline void destroy_compound_gigantic_page(struct page *page,
> unsigned int order) { }
> #endif
>
> -static void update_and_free_page(struct hstate *h, struct page *page)
> +static int update_and_free_page(struct hstate *h, struct page *page)
> + __releases(&hugetlb_lock) __acquires(&hugetlb_lock)
> {
> int i;
> struct page *subpage = page;
> + int nid = page_to_nid(page);
>
> if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
> - return;
> + return 0;
>
> h->nr_huge_pages--;
> - h->nr_huge_pages_node[page_to_nid(page)]--;
> + h->nr_huge_pages_node[nid]--;
> + VM_BUG_ON_PAGE(hugetlb_cgroup_from_page(page), page);
> + VM_BUG_ON_PAGE(hugetlb_cgroup_from_page_rsvd(page), page);
> + set_page_refcounted(page);
> + set_compound_page_dtor(page, NULL_COMPOUND_DTOR);
> +
> + /*
> + * If the vmemmap pages associated with the HugeTLB page can be
> + * optimized or the page is gigantic, we might block in
> + * alloc_huge_page_vmemmap() or free_gigantic_page(). In both
> + * cases, drop the hugetlb_lock.
> + */
> + if (free_vmemmap_pages_per_hpage(h) || hstate_is_gigantic(h))
> + spin_unlock(&hugetlb_lock);
> +
> + if (alloc_huge_page_vmemmap(h, page)) {
> + spin_lock(&hugetlb_lock);
> + INIT_LIST_HEAD(&page->lru);
> + set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
> + h->nr_huge_pages++;
> + h->nr_huge_pages_node[nid]++;
> +
> + /*
> + * If we cannot allocate vmemmap pages, just refuse to free the
> + * page and put the page back on the hugetlb free list and treat
> + * as a surplus page.
> + */
> + h->surplus_huge_pages++;
> + h->surplus_huge_pages_node[nid]++;
> +
> + /*
> + * The refcount can be perfectly increased by memory-failure or
> + * soft_offline handlers.
> + */
> + if (likely(put_page_testzero(page))) {
> + arch_clear_hugepage_flags(page);
> + enqueue_huge_page(h, page);
> + }
> +
> + return -ENOMEM;
> + }
> +
> for (i = 0; i < pages_per_huge_page(h);
> i++, subpage = mem_map_next(subpage, page, i)) {
> subpage->flags &= ~(1 << PG_locked | 1 << PG_error |
> @@ -1321,22 +1364,18 @@ static void update_and_free_page(struct hstate *h, struct page *page)
> 1 << PG_active | 1 << PG_private |
> 1 << PG_writeback);
> }
> - VM_BUG_ON_PAGE(hugetlb_cgroup_from_page(page), page);
> - VM_BUG_ON_PAGE(hugetlb_cgroup_from_page_rsvd(page), page);
> - set_compound_page_dtor(page, NULL_COMPOUND_DTOR);
> - set_page_refcounted(page);
> +
> if (hstate_is_gigantic(h)) {
> - /*
> - * Temporarily drop the hugetlb_lock, because
> - * we might block in free_gigantic_page().
> - */
> - spin_unlock(&hugetlb_lock);
> destroy_compound_gigantic_page(page, huge_page_order(h));
> free_gigantic_page(page, huge_page_order(h));
> - spin_lock(&hugetlb_lock);
> } else {
> __free_pages(page, huge_page_order(h));
> }
> +
> + if (free_vmemmap_pages_per_hpage(h) || hstate_is_gigantic(h))
> + spin_lock(&hugetlb_lock);
> +
> + return 0;
> }
>
> struct hstate *size_to_hstate(unsigned long size)
> @@ -1404,9 +1443,9 @@ static void __free_huge_page(struct page *page)
> } else if (h->surplus_huge_pages_node[nid]) {
> /* remove the page from active list */
> list_del(&page->lru);
> - update_and_free_page(h, page);
> h->surplus_huge_pages--;
> h->surplus_huge_pages_node[nid]--;
> + update_and_free_page(h, page);
> } else {
> arch_clear_hugepage_flags(page);
> enqueue_huge_page(h, page);
> @@ -1447,7 +1486,7 @@ void free_huge_page(struct page *page)
> /*
> * Defer freeing if in non-task context to avoid hugetlb_lock deadlock.
> */
> - if (!in_task()) {
> + if (!in_atomic()) {
> /*
> * Only call schedule_work() if hpage_freelist is previously
> * empty. Otherwise, schedule_work() had been called but the
> @@ -1699,8 +1738,7 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed,
> h->surplus_huge_pages--;
> h->surplus_huge_pages_node[node]--;
> }
> - update_and_free_page(h, page);
> - ret = 1;
> + ret = !update_and_free_page(h, page);
> break;
> }
> }
> @@ -1713,10 +1751,14 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed,
> * nothing for in-use hugepages and non-hugepages.
> * This function returns values like below:
> *
> - * -EBUSY: failed to dissolved free hugepages or the hugepage is in-use
> - * (allocated or reserved.)
> - * 0: successfully dissolved free hugepages or the page is not a
> - * hugepage (considered as already dissolved)
> + * -ENOMEM: failed to allocate vmemmap pages to free the freed hugepages
> + * when the system is under memory pressure and the feature of
> + * freeing unused vmemmap pages associated with each hugetlb page
> + * is enabled.
> + * -EBUSY: failed to dissolved free hugepages or the hugepage is in-use
> + * (allocated or reserved.)
> + * 0: successfully dissolved free hugepages or the page is not a
> + * hugepage (considered as already dissolved)
> */
> int dissolve_free_huge_page(struct page *page)
> {
> @@ -1771,8 +1813,12 @@ int dissolve_free_huge_page(struct page *page)
> h->free_huge_pages--;
> h->free_huge_pages_node[nid]--;
> h->max_huge_pages--;
> - update_and_free_page(h, head);
> - rc = 0;
> + rc = update_and_free_page(h, head);
> + if (rc) {
> + h->surplus_huge_pages--;
> + h->surplus_huge_pages_node[nid]--;
> + h->max_huge_pages++;
> + }
> }
> out:
> spin_unlock(&hugetlb_lock);
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index 0209b736e0b4..f7ab3d99250a 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -181,21 +181,31 @@
> #define RESERVE_VMEMMAP_NR 2U
> #define RESERVE_VMEMMAP_SIZE (RESERVE_VMEMMAP_NR << PAGE_SHIFT)
>
> -/*
> - * How many vmemmap pages associated with a HugeTLB page that can be freed
> - * to the buddy allocator.
> - *
> - * Todo: Returns zero for now, which means the feature is disabled. We will
> - * enable it once all the infrastructure is there.
> - */
> -static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
> +static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h)
> {
> - return 0;
> + return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT;
> }
>
> -static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h)
> +int alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
> {
> - return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT;
> + unsigned long vmemmap_addr = (unsigned long)head;
> + unsigned long vmemmap_end, vmemmap_reuse;
> +
> + if (!free_vmemmap_pages_per_hpage(h))
> + return 0;
> +
> + vmemmap_addr += RESERVE_VMEMMAP_SIZE;
> + vmemmap_end = vmemmap_addr + free_vmemmap_pages_size_per_hpage(h);
> + vmemmap_reuse = vmemmap_addr - PAGE_SIZE;
> + /*
> + * The pages which the vmemmap virtual address range [@vmemmap_addr,
> + * @vmemmap_end) are mapped to are freed to the buddy allocator, and
> + * the range is mapped to the page which @vmemmap_reuse is mapped to.
> + * When a HugeTLB page is freed to the buddy allocator, previously
> + * discarded vmemmap pages must be allocated and remapping.
> + */
> + return vmemmap_remap_alloc(vmemmap_addr, vmemmap_end, vmemmap_reuse,
> + GFP_KERNEL | __GFP_NORETRY | __GFP_THISNODE);
> }
>
> void free_huge_page_vmemmap(struct hstate *h, struct page *head)
> diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
> index 6923f03534d5..a37771b0b82a 100644
> --- a/mm/hugetlb_vmemmap.h
> +++ b/mm/hugetlb_vmemmap.h
> @@ -11,10 +11,33 @@
> #include <linux/hugetlb.h>
>
> #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> +int alloc_huge_page_vmemmap(struct hstate *h, struct page *head);
> void free_huge_page_vmemmap(struct hstate *h, struct page *head);
> +
> +/*
> + * How many vmemmap pages associated with a HugeTLB page that can be freed
> + * to the buddy allocator.
> + *
> + * Todo: Returns zero for now, which means the feature is disabled. We will
> + * enable it once all the infrastructure is there.
> + */
> +static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
> +{
> + return 0;
> +}
> #else
> +static inline int alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
> +{
> + return 0;
> +}
> +
> static inline void free_huge_page_vmemmap(struct hstate *h, struct page *head)
> {
> }
> +
> +static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
> +{
> + return 0;
> +}
> #endif /* CONFIG_HUGETLB_PAGE_FREE_VMEMMAP */
> #endif /* _LINUX_HUGETLB_VMEMMAP_H */
> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> index d3076a7a3783..60fc6cd6cd23 100644
> --- a/mm/sparse-vmemmap.c
> +++ b/mm/sparse-vmemmap.c
> @@ -40,7 +40,8 @@
> * @remap_pte: called for each lowest-level entry (PTE).
> * @reuse_page: the page which is reused for the tail vmemmap pages.
> * @reuse_addr: the virtual address of the @reuse_page page.
> - * @vmemmap_pages: the list head of the vmemmap pages that can be freed.
> + * @vmemmap_pages: the list head of the vmemmap pages that can be freed
> + * or is mapped from.
> */
> struct vmemmap_remap_walk {
> void (*remap_pte)(pte_t *pte, unsigned long addr,
> @@ -237,6 +238,78 @@ void vmemmap_remap_free(unsigned long start, unsigned long end,
> free_vmemmap_page_list(&vmemmap_pages);
> }
>
> +static void vmemmap_restore_pte(pte_t *pte, unsigned long addr,
> + struct vmemmap_remap_walk *walk)
> +{
> + pgprot_t pgprot = PAGE_KERNEL;
> + struct page *page;
> + void *to;
> +
> + BUG_ON(pte_page(*pte) != walk->reuse_page);
> +
> + page = list_first_entry(walk->vmemmap_pages, struct page, lru);
> + list_del(&page->lru);
> + to = page_to_virt(page);
> + copy_page(to, (void *)walk->reuse_addr);
> +
> + set_pte_at(&init_mm, addr, pte, mk_pte(page, pgprot));
> +}
> +
> +static int alloc_vmemmap_page_list(unsigned long start, unsigned long end,
> + gfp_t gfp_mask, struct list_head *list)
> +{
> + unsigned long nr_pages = (end - start) >> PAGE_SHIFT;
> + int nid = page_to_nid((struct page *)start);
> + struct page *page, *next;
> +
> + while (nr_pages--) {
> + page = alloc_pages_node(nid, gfp_mask, 0);
> + if (!page)
> + goto out;
> + list_add_tail(&page->lru, list);
> + }
> +
> + return 0;
> +out:
> + list_for_each_entry_safe(page, next, list, lru)
> + __free_pages(page, 0);
> + return -ENOMEM;
> +}
> +
> +/**
> + * vmemmap_remap_alloc - remap the vmemmap virtual address range [@start, end)
> + * to the page which is from the @vmemmap_pages
> + * respectively.
> + * @start: start address of the vmemmap virtual address range that we want
> + * to remap.
> + * @end: end address of the vmemmap virtual address range that we want to
> + * remap.
> + * @reuse: reuse address.
> + * @gpf_mask: GFP flag for allocating vmemmap pages.
> + */
> +int vmemmap_remap_alloc(unsigned long start, unsigned long end,
> + unsigned long reuse, gfp_t gfp_mask)
> +{
> + LIST_HEAD(vmemmap_pages);
> + struct vmemmap_remap_walk walk = {
> + .remap_pte = vmemmap_restore_pte,
> + .reuse_addr = reuse,
> + .vmemmap_pages = &vmemmap_pages,
> + };
> +
> + /* See the comment in the vmemmap_remap_free(). */
> + BUG_ON(start - reuse != PAGE_SIZE);
> +
> + might_sleep_if(gfpflags_allow_blocking(gfp_mask));
> +
> + if (alloc_vmemmap_page_list(start, end, gfp_mask, &vmemmap_pages))
> + return -ENOMEM;
> +
> + vmemmap_remap_range(reuse, end, &walk);
> +
> + return 0;
> +}
> +
> /*
> * Allocate a block of memory to be used to back the virtual memory map
> * or to back the page tables that are used to create the mapping.
> --
> 2.11.0
>

2021-03-04 06:24:42

by Chen Huang

Subject: Re: [PATCH v17 0/9] Free some vmemmap pages of HugeTLB page

在 2021/2/25 21:21, Muchun Song 写道:
> Hi all,
>
> This patch series will free some vmemmap pages(struct page structures)
> associated with each hugetlbpage when preallocated to save memory.
>
> In order to reduce the difficulty of the first version of code review.
>>From this version, we disable PMD/huge page mapping of vmemmap if this
> feature was enabled. This accutualy eliminate a bunch of the complex code
> doing page table manipulation. When this patch series is solid, we cam add
> the code of vmemmap page table manipulation in the future.
>
> The struct page structures (page structs) are used to describe a physical
> page frame. By default, there is a one-to-one mapping from a page frame to
> it's corresponding page struct.
>
> The HugeTLB pages consist of multiple base page size pages and is supported
> by many architectures. See hugetlbpage.rst in the Documentation directory
> for more details. On the x86 architecture, HugeTLB pages of size 2MB and 1GB
> are currently supported. Since the base page size on x86 is 4KB, a 2MB
> HugeTLB page consists of 512 base pages and a 1GB HugeTLB page consists of
> 4096 base pages. For each base page, there is a corresponding page struct.
>
> Within the HugeTLB subsystem, only the first 4 page structs are used to
> contain unique information about a HugeTLB page. HUGETLB_CGROUP_MIN_ORDER
> provides this upper limit. The only 'useful' information in the remaining
> page structs is the compound_head field, and this field is the same for all
> tail pages.
>
> By removing redundant page structs for HugeTLB pages, memory can returned to
> the buddy allocator for other uses.
>
> When the system boot up, every 2M HugeTLB has 512 struct page structs which
> size is 8 pages(sizeof(struct page) * 512 / PAGE_SIZE).
>
> HugeTLB struct pages(8 pages) page frame(8 pages)
> +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+
> | | | 0 | -------------> | 0 |
> | | +-----------+ +-----------+
> | | | 1 | -------------> | 1 |
> | | +-----------+ +-----------+
> | | | 2 | -------------> | 2 |
> | | +-----------+ +-----------+
> | | | 3 | -------------> | 3 |
> | | +-----------+ +-----------+
> | | | 4 | -------------> | 4 |
> | 2MB | +-----------+ +-----------+
> | | | 5 | -------------> | 5 |
> | | +-----------+ +-----------+
> | | | 6 | -------------> | 6 |
> | | +-----------+ +-----------+
> | | | 7 | -------------> | 7 |
> | | +-----------+ +-----------+
> | |
> | |
> | |
> +-----------+
>
> The value of page->compound_head is the same for all tail pages. The first
> page of page structs (page 0) associated with the HugeTLB page contains the 4
> page structs necessary to describe the HugeTLB. The only use of the remaining
> pages of page structs (page 1 to page 7) is to point to page->compound_head.
> Therefore, we can remap pages 2 to 7 to page 1. Only 2 pages of page structs
> will be used for each HugeTLB page. This will allow us to free the remaining
> 6 pages to the buddy allocator.
>
> Here is how things look after remapping.
>
> HugeTLB struct pages(8 pages) page frame(8 pages)
> +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+
> | | | 0 | -------------> | 0 |
> | | +-----------+ +-----------+
> | | | 1 | -------------> | 1 |
> | | +-----------+ +-----------+
> | | | 2 | ----------------^ ^ ^ ^ ^ ^
> | | +-----------+ | | | | |
> | | | 3 | ------------------+ | | | |
> | | +-----------+ | | | |
> | | | 4 | --------------------+ | | |
> | 2MB | +-----------+ | | |
> | | | 5 | ----------------------+ | |
> | | +-----------+ | |
> | | | 6 | ------------------------+ |
> | | +-----------+ |
> | | | 7 | --------------------------+
> | | +-----------+
> | |
> | |
> | |
> +-----------+
>
> When a HugeTLB is freed to the buddy system, we should allocate 6 pages for
> vmemmap pages and restore the previous mapping relationship.
>
> Apart from 2MB HugeTLB page, we also have 1GB HugeTLB page. It is similar
> to the 2MB HugeTLB page. We also can use this approach to free the vmemmap
> pages.
>
> In this case, for the 1GB HugeTLB page, we can save 4094 pages. This is a
> very substantial gain. On our server, run some SPDK/QEMU applications which
> will use 1024GB hugetlbpage. With this feature enabled, we can save ~16GB
> (1G hugepage)/~12GB (2MB hugepage) memory.
>
> Because there are vmemmap page tables reconstruction on the freeing/allocating
> path, it increases some overhead. Here are some overhead analysis.
>
> 1) Allocating 10240 2MB hugetlb pages.
>
> a) With this patch series applied:
> # time echo 10240 > /proc/sys/vm/nr_hugepages
>
> real 0m0.166s
> user 0m0.000s
> sys 0m0.166s
>
> # bpftrace -e 'kprobe:alloc_fresh_huge_page { @start[tid] = nsecs; }
> kretprobe:alloc_fresh_huge_page /@start[tid]/ { @latency = hist(nsecs -
> @start[tid]); delete(@start[tid]); }'
> Attaching 2 probes...
>
> @latency:
> [8K, 16K) 5476 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
> [16K, 32K) 4760 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ |
> [32K, 64K) 4 | |
>
> b) Without this patch series:
> # time echo 10240 > /proc/sys/vm/nr_hugepages
>
> real 0m0.067s
> user 0m0.000s
> sys 0m0.067s
>
> # bpftrace -e 'kprobe:alloc_fresh_huge_page { @start[tid] = nsecs; }
> kretprobe:alloc_fresh_huge_page /@start[tid]/ { @latency = hist(nsecs -
> @start[tid]); delete(@start[tid]); }'
> Attaching 2 probes...
>
> @latency:
> [4K, 8K) 10147 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
> [8K, 16K) 93 | |
>
> Summarize: this feature is about ~2x slower than before.
>
> 2) Freeing 10240 2MB hugetlb pages.
>
> a) With this patch series applied:
> # time echo 0 > /proc/sys/vm/nr_hugepages
>
> real 0m0.213s
> user 0m0.000s
> sys 0m0.213s
>
> # bpftrace -e 'kprobe:free_pool_huge_page { @start[tid] = nsecs; }
> kretprobe:free_pool_huge_page /@start[tid]/ { @latency = hist(nsecs -
> @start[tid]); delete(@start[tid]); }'
> Attaching 2 probes...
>
> @latency:
> [8K, 16K) 6 | |
> [16K, 32K) 10227 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
> [32K, 64K) 7 | |
>
> b) Without this patch series:
> # time echo 0 > /proc/sys/vm/nr_hugepages
>
> real 0m0.081s
> user 0m0.000s
> sys 0m0.081s
>
> # bpftrace -e 'kprobe:free_pool_huge_page { @start[tid] = nsecs; }
> kretprobe:free_pool_huge_page /@start[tid]/ { @latency = hist(nsecs -
> @start[tid]); delete(@start[tid]); }'
> Attaching 2 probes...
>
> @latency:
> [4K, 8K) 6805 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
> [8K, 16K) 3427 |@@@@@@@@@@@@@@@@@@@@@@@@@@ |
> [16K, 32K) 8 | |
>
> Summarize: The overhead of __free_hugepage is about ~2-3x slower than before.
>
> Although the overhead has increased, the overhead is not significant. Like Mike
> said, "However, remember that the majority of use cases create hugetlb pages at
> or shortly after boot time and add them to the pool. So, additional overhead is
> at pool creation time. There is no change to 'normal run time' operations of
> getting a page from or returning a page to the pool (think page fault/unmap)".
>
> Despite the overhead and in addition to the memory gains from this series. The
> following data is obtained by Joao Martins. Very thanks to his effort.
>
> There's an additional benefit which is page (un)pinners will see an improvement
> and Joao presumes because there are fewer memmap pages and thus the tail/head
> pages are staying in cache more often.
>
> Out of the box Joao saw (when comparing linux-next against linux-next + this series)
> with gup_test and pinning a 16G hugetlb file (with 1G pages):
>
> get_user_pages(): ~32k -> ~9k
> unpin_user_pages(): ~75k -> ~70k
>
> Usually any tight loop fetching compound_head(), or reading tail pages data (e.g.
> compound_head) benefit a lot. There's some unpinning inefficiencies Joao was
> fixing[0], but with that in added it shows even more:
>
> unpin_user_pages(): ~27k -> ~3.8k
>
> [0] https://lore.kernel.org/linux-mm/[email protected]/
>
> Todo:
> - Free all of the tail vmemmap pages
> Now for the 2MB HugrTLB page, we only free 6 vmemmap pages. we really can
> free 7 vmemmap pages. In this case, we can see 8 of the 512 struct page
> structures has beed set PG_head flag. If we can adjust compound_head()
> slightly and make compound_head() return the real head struct page when
> the parameter is the tail struct page but with PG_head flag set.
>
> In order to make the code evolution route clearer. This feature can can be
> a separate patch after this patchset is solid.
>
> - Support for other architectures (e.g. aarch64).
> - Enable PMD/huge page mapping of vmemmap even if this feature was enabled.

Tested-by: Chen Huang <[email protected]>

We are interested in this patch and have tested the patch for x86. Also we made a simple
modification in arm64 and tested for the patch.

1. In x86, we set the total memory of 70G, and use 32G for hugepages then got the result:
------------------------------------------------------------------------------------------------
2M page | 1G page |
----------------------|------------------------|----------------------|------------------------|
enable | disable | enable | disable |
----------------------|------------------------|----------------------|------------------------|
total | used | free | total | used | free |total | used | free | total | used | free |
70855 | 33069 | 37786| 70473 | 33068 | 37405 |70983 | 33068 | 37914| 70473 | 33068 | 37405 |
------------------------------------------------------------------------------------------------
The result is that for 2M hugepage, we can save 382M memory which is correspoinding to the expected
384M memory. For 1G hugepage, we can save 510M memory which is correspoinding to the expected 512M
memory.

2. In arm64, the hack modification is shown below[1]. We set the total memory of 40G, and use 10G
for hugepages then got the result:
------------------------------------------------------------------------------------------------
2M page | 1G page |
----------------------|------------------------|----------------------|------------------------|
enable | disable | enable | disable |
----------------------|------------------------|----------------------|------------------------|
total | used | free | total | used | free |total | used | free | total | used | free |
39,739 | 10279 |29,460| 39579 | 10278 | 29,301‬|39,699 | 10279 |29,420| 39579 | 10278 | 29,301|
------------------------------------------------------------------------------------------------
The result is that for 2M hugepage, we can save 119M memory which is correspoinding to the expected
120M memory. For 1G hugepage, we can save 159M memory which is correspoinding to the expected 160M
memory.

3. Also we found that when we free and realloc the 1G hugepages, as the vmemmap need realloc pages before
freeing the hugepages, it will decrese the chance to get hugepages. Because the freeed hugepages has
returned to the buddy system and may be used for vmemmap pages.
We think failing to alloc hugepages is normal so this is fine.

[1]: support arm64
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index a0a41e6c1307..c150c6e6e20c 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -1091,7 +1091,7 @@ static void free_empty_tables(unsigned long addr, unsigned long end,
#endif

#ifdef CONFIG_SPARSEMEM_VMEMMAP
-#if !ARM64_SWAPPER_USES_SECTION_MAPS
+#if !ARM64_SWAPPER_USES_SECTION_MAPS || defined(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP)
int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
struct vmem_altmap *altmap)
{
diff --git a/fs/Kconfig b/fs/Kconfig
index de87f234f1e9..fa46c9dfa256 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -239,9 +239,8 @@ config HUGETLB_PAGE

config HUGETLB_PAGE_FREE_VMEMMAP
def_bool HUGETLB_PAGE
- depends on X86_64
+ depends on (X86_64 && HAVE_BOOTMEM_INFO_NODE) || ARM64
depends on SPARSEMEM_VMEMMAP
- depends on HAVE_BOOTMEM_INFO_NODE

config MEMFD_CREATE
def_bool TMPFS || HUGETLBFS
diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
index 60fc6cd6cd23..b25d3b895eaa 100644
--- a/mm/sparse-vmemmap.c
+++ b/mm/sparse-vmemmap.c
@@ -165,7 +165,11 @@ static void vmemmap_remap_range(unsigned long start, unsigned long end,
static inline void free_vmemmap_page(struct page *page)
{
if (PageReserved(page))
+#if defined(CONFIG_ARM64) && defined(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP)
+ free_reserved_page(page);
+#else
free_bootmem_page(page);
+#endif
else
__free_page(page);
}

2021-03-04 08:08:24

by Mike Kravetz

Subject: Re: [PATCH v17 4/9] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page

On 2/25/21 5:21 AM, Muchun Song wrote:
> When we free a HugeTLB page to the buddy allocator, we should allocate
> the vmemmap pages associated with it. But we may cannot allocate vmemmap
> pages when the system is under memory pressure, in this case, we just
> refuse to free the HugeTLB page instead of looping forever trying to
> allocate the pages. This changes some behavior (list below) on some
> cassociated with it.orner cases.

Suggest rewording this as:

When we free a HugeTLB page to the buddy allocator, we need to allocate
the vmemmap pages associated with it. However, we may not be able to
allocate the vmemmap pages when the system is under memory pressure. In
this case, we just refuse to free the HugeTLB page. This changes behavior
in some corner cases as listed below:

>
> 1) Failing to free a huge page triggered by the user (decrease nr_pages).
>
> Need try again later by the user.

User needs to try again later.
>
> 2) Failing to free a surplus huge page when freed by the application.
>
> Try again later when freeing a huge page next time.
>
> 3) Failing to dissolve a free huge page on ZONE_MOVABLE via
> offline_pages().
>
> This is a bit unfortunate if we have plenty of ZONE_MOVABLE memory
> but are low on kernel memory. For example, migration of huge pages
> would still work, however, dissolving the free page does not work.
> This is a corner cases. When the system is that much under memory
> pressure, offlining/unplug can be expected to fail. This is
> unfortunate because it prevents from the memory offlining which
> shouldn't happen for movable zones. People depending on the memory
> hotplug and movable zone should carefuly consider whether savings
> on unmovable memory are worth losing their hotplug functionality
> in some situations.
>

Possible wording change:
This can happen when we have plenty of ZONE_MOVABLE memory, but
not enough kernel memory to allocate vmemmmap pages. We may even
be able to migrate huge page contents, but will not be able to
dissolve the source huge page. This will prevent an offline
operation and is unfortunate as memory offlining is expected to
succeed on movable zones. Users that depend on memory hotplug
to succeed for movable zones should carefully consider whether the
memory savings gained from this feature are worth the risk of
possibly not being able to offline memory in certain situations.

> 4) Failing to dissolve a huge page on CMA/ZONE_MOVABLE via
> alloc_contig_range() - once we have that handling in place. Mainly
> affects CMA and virtio-mem.
>
> Similar to 3). virito-mem will handle migration errors gracefully.
> CMA might be able to fallback on other free areas within the CMA
> region.
>
> Vmemmap pages are allocated from the page freeing context. In order for
> those allocations to be not disruptive (e.g. trigger oom killer)
> __GFP_NORETRY is used. hugetlb_lock is dropped for the allocation
> because a non sleeping allocation would be too fragile and it could fail
> too easily under memory pressure. GFP_ATOMIC or other modes to access
> memory reserves is not used because we want to prevent consuming
> reserves under heavy hugetlb freeing.
>
> Signed-off-by: Muchun Song <[email protected]>
> ---
> Documentation/admin-guide/mm/hugetlbpage.rst | 8 +++
> include/linux/mm.h | 2 +
> mm/hugetlb.c | 92 +++++++++++++++++++++-------
> mm/hugetlb_vmemmap.c | 32 ++++++----
> mm/hugetlb_vmemmap.h | 23 +++++++
> mm/sparse-vmemmap.c | 75 ++++++++++++++++++++++-
> 6 files changed, 197 insertions(+), 35 deletions(-)
>
> diff --git a/Documentation/admin-guide/mm/hugetlbpage.rst b/Documentation/admin-guide/mm/hugetlbpage.rst
> index f7b1c7462991..6988895d09a8 100644
> --- a/Documentation/admin-guide/mm/hugetlbpage.rst
> +++ b/Documentation/admin-guide/mm/hugetlbpage.rst
> @@ -60,6 +60,10 @@ HugePages_Surp
> the pool above the value in ``/proc/sys/vm/nr_hugepages``. The
> maximum number of surplus huge pages is controlled by
> ``/proc/sys/vm/nr_overcommit_hugepages``.
> + Note: When the feature of freeing unused vmemmap pages associated
> + with each hugetlb page is enabled, the number of surplus huge pages
> + may be temporarily larger than the maximum number of surplus huge
> + pages when the system is under memory pressure.
> Hugepagesize
> is the default hugepage size (in Kb).
> Hugetlb
> @@ -80,6 +84,10 @@ returned to the huge page pool when freed by a task. A user with root
> privileges can dynamically allocate more or free some persistent huge pages
> by increasing or decreasing the value of ``nr_hugepages``.
>
> +Note: When the feature of freeing unused vmemmap pages associated with each
> +hugetlb page is enabled, we can fail to free the huge pages triggered by
> +the user when ths system is under memory pressure. Please try again later.
> +
> Pages that are used as huge pages are reserved inside the kernel and cannot
> be used for other purposes. Huge pages cannot be swapped out under
> memory pressure.
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 4ddfc31f21c6..77693c944a36 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2973,6 +2973,8 @@ static inline void print_vma_addr(char *prefix, unsigned long rip)
>
> void vmemmap_remap_free(unsigned long start, unsigned long end,
> unsigned long reuse);
> +int vmemmap_remap_alloc(unsigned long start, unsigned long end,
> + unsigned long reuse, gfp_t gfp_mask);
>
> void *sparse_buffer_alloc(unsigned long size);
> struct page * __populate_section_memmap(unsigned long pfn,
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 43fed6785322..b6e4e3f31ad2 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1304,16 +1304,59 @@ static inline void destroy_compound_gigantic_page(struct page *page,
> unsigned int order) { }
> #endif
>
> -static void update_and_free_page(struct hstate *h, struct page *page)
> +static int update_and_free_page(struct hstate *h, struct page *page)
> + __releases(&hugetlb_lock) __acquires(&hugetlb_lock)
> {
> int i;
> struct page *subpage = page;
> + int nid = page_to_nid(page);
>
> if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
> - return;
> + return 0;
>
> h->nr_huge_pages--;
> - h->nr_huge_pages_node[page_to_nid(page)]--;
> + h->nr_huge_pages_node[nid]--;
> + VM_BUG_ON_PAGE(hugetlb_cgroup_from_page(page), page);
> + VM_BUG_ON_PAGE(hugetlb_cgroup_from_page_rsvd(page), page);
> + set_page_refcounted(page);
> + set_compound_page_dtor(page, NULL_COMPOUND_DTOR);
> +
> + /*
> + * If the vmemmap pages associated with the HugeTLB page can be
> + * optimized or the page is gigantic, we might block in
> + * alloc_huge_page_vmemmap() or free_gigantic_page(). In both
> + * cases, drop the hugetlb_lock.
> + */
> + if (free_vmemmap_pages_per_hpage(h) || hstate_is_gigantic(h))
> + spin_unlock(&hugetlb_lock);
> +
> + if (alloc_huge_page_vmemmap(h, page)) {
> + spin_lock(&hugetlb_lock);
> + INIT_LIST_HEAD(&page->lru);
> + set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
> + h->nr_huge_pages++;
> + h->nr_huge_pages_node[nid]++;
> +
> + /*
> + * If we cannot allocate vmemmap pages, just refuse to free the
> + * page and put the page back on the hugetlb free list and treat
> + * as a surplus page.
> + */
> + h->surplus_huge_pages++;
> + h->surplus_huge_pages_node[nid]++;
> +
> + /*
> + * The refcount can be perfectly increased by memory-failure or

Reword The refcount can possibly be increased by memory-failure or
soft_offline handlers.

> + * soft_offline handlers.
> + */
> + if (likely(put_page_testzero(page))) {
> + arch_clear_hugepage_flags(page);
> + enqueue_huge_page(h, page);
> + }
> +
> + return -ENOMEM;
> + }
> +
> for (i = 0; i < pages_per_huge_page(h);
> i++, subpage = mem_map_next(subpage, page, i)) {
> subpage->flags &= ~(1 << PG_locked | 1 << PG_error |
> @@ -1321,22 +1364,18 @@ static void update_and_free_page(struct hstate *h, struct page *page)
> 1 << PG_active | 1 << PG_private |
> 1 << PG_writeback);
> }
> - VM_BUG_ON_PAGE(hugetlb_cgroup_from_page(page), page);
> - VM_BUG_ON_PAGE(hugetlb_cgroup_from_page_rsvd(page), page);
> - set_compound_page_dtor(page, NULL_COMPOUND_DTOR);
> - set_page_refcounted(page);
> +
> if (hstate_is_gigantic(h)) {
> - /*
> - * Temporarily drop the hugetlb_lock, because
> - * we might block in free_gigantic_page().
> - */
> - spin_unlock(&hugetlb_lock);
> destroy_compound_gigantic_page(page, huge_page_order(h));
> free_gigantic_page(page, huge_page_order(h));
> - spin_lock(&hugetlb_lock);
> } else {
> __free_pages(page, huge_page_order(h));
> }
> +
> + if (free_vmemmap_pages_per_hpage(h) || hstate_is_gigantic(h))
> + spin_lock(&hugetlb_lock);
> +
> + return 0;
> }
>
> struct hstate *size_to_hstate(unsigned long size)
> @@ -1404,9 +1443,9 @@ static void __free_huge_page(struct page *page)
> } else if (h->surplus_huge_pages_node[nid]) {
> /* remove the page from active list */
> list_del(&page->lru);
> - update_and_free_page(h, page);
> h->surplus_huge_pages--;
> h->surplus_huge_pages_node[nid]--;
> + update_and_free_page(h, page);
> } else {
> arch_clear_hugepage_flags(page);
> enqueue_huge_page(h, page);
> @@ -1447,7 +1486,7 @@ void free_huge_page(struct page *page)
> /*
> * Defer freeing if in non-task context to avoid hugetlb_lock deadlock.
> */
> - if (!in_task()) {
> + if (!in_atomic()) {

That should be "if (in_atomic()) instead of "if (!in_atomic())"

Do note that there is an ongoing discussion about calling free_huge_page
in various contexts.

https://lore.kernel.org/linux-mm/[email protected]/

This discussion/issue is independent of this patch. Since that issue
deals with existing code, we will need to come up with a solution there
first. A solution there may impact how free_huge_page is structured and
may impact this patch.

The in_atomic() check is insufficient to handle all cases. It is better
than !in_task(), but still does not cover all cases.

The rest of the patch looks good to me.
--
Mike Kravetz

> /*
> * Only call schedule_work() if hpage_freelist is previously
> * empty. Otherwise, schedule_work() had been called but the
> @@ -1699,8 +1738,7 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed,
> h->surplus_huge_pages--;
> h->surplus_huge_pages_node[node]--;
> }
> - update_and_free_page(h, page);
> - ret = 1;
> + ret = !update_and_free_page(h, page);
> break;
> }
> }
> @@ -1713,10 +1751,14 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed,
> * nothing for in-use hugepages and non-hugepages.
> * This function returns values like below:
> *
> - * -EBUSY: failed to dissolved free hugepages or the hugepage is in-use
> - * (allocated or reserved.)
> - * 0: successfully dissolved free hugepages or the page is not a
> - * hugepage (considered as already dissolved)
> + * -ENOMEM: failed to allocate vmemmap pages to free the freed hugepages
> + * when the system is under memory pressure and the feature of
> + * freeing unused vmemmap pages associated with each hugetlb page
> + * is enabled.
> + * -EBUSY: failed to dissolved free hugepages or the hugepage is in-use
> + * (allocated or reserved.)
> + * 0: successfully dissolved free hugepages or the page is not a
> + * hugepage (considered as already dissolved)
> */
> int dissolve_free_huge_page(struct page *page)
> {
> @@ -1771,8 +1813,12 @@ int dissolve_free_huge_page(struct page *page)
> h->free_huge_pages--;
> h->free_huge_pages_node[nid]--;
> h->max_huge_pages--;
> - update_and_free_page(h, head);
> - rc = 0;
> + rc = update_and_free_page(h, head);
> + if (rc) {
> + h->surplus_huge_pages--;
> + h->surplus_huge_pages_node[nid]--;
> + h->max_huge_pages++;
> + }
> }
> out:
> spin_unlock(&hugetlb_lock);
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index 0209b736e0b4..f7ab3d99250a 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -181,21 +181,31 @@
> #define RESERVE_VMEMMAP_NR 2U
> #define RESERVE_VMEMMAP_SIZE (RESERVE_VMEMMAP_NR << PAGE_SHIFT)
>
> -/*
> - * How many vmemmap pages associated with a HugeTLB page that can be freed
> - * to the buddy allocator.
> - *
> - * Todo: Returns zero for now, which means the feature is disabled. We will
> - * enable it once all the infrastructure is there.
> - */
> -static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
> +static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h)
> {
> - return 0;
> + return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT;
> }
>
> -static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h)
> +int alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
> {
> - return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT;
> + unsigned long vmemmap_addr = (unsigned long)head;
> + unsigned long vmemmap_end, vmemmap_reuse;
> +
> + if (!free_vmemmap_pages_per_hpage(h))
> + return 0;
> +
> + vmemmap_addr += RESERVE_VMEMMAP_SIZE;
> + vmemmap_end = vmemmap_addr + free_vmemmap_pages_size_per_hpage(h);
> + vmemmap_reuse = vmemmap_addr - PAGE_SIZE;
> + /*
> + * The pages which the vmemmap virtual address range [@vmemmap_addr,
> + * @vmemmap_end) are mapped to are freed to the buddy allocator, and
> + * the range is mapped to the page which @vmemmap_reuse is mapped to.
> + * When a HugeTLB page is freed to the buddy allocator, previously
> + * discarded vmemmap pages must be allocated and remapping.
> + */
> + return vmemmap_remap_alloc(vmemmap_addr, vmemmap_end, vmemmap_reuse,
> + GFP_KERNEL | __GFP_NORETRY | __GFP_THISNODE);
> }
>
> void free_huge_page_vmemmap(struct hstate *h, struct page *head)
> diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
> index 6923f03534d5..a37771b0b82a 100644
> --- a/mm/hugetlb_vmemmap.h
> +++ b/mm/hugetlb_vmemmap.h
> @@ -11,10 +11,33 @@
> #include <linux/hugetlb.h>
>
> #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> +int alloc_huge_page_vmemmap(struct hstate *h, struct page *head);
> void free_huge_page_vmemmap(struct hstate *h, struct page *head);
> +
> +/*
> + * How many vmemmap pages associated with a HugeTLB page that can be freed
> + * to the buddy allocator.
> + *
> + * Todo: Returns zero for now, which means the feature is disabled. We will
> + * enable it once all the infrastructure is there.
> + */
> +static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
> +{
> + return 0;
> +}
> #else
> +static inline int alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
> +{
> + return 0;
> +}
> +
> static inline void free_huge_page_vmemmap(struct hstate *h, struct page *head)
> {
> }
> +
> +static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
> +{
> + return 0;
> +}
> #endif /* CONFIG_HUGETLB_PAGE_FREE_VMEMMAP */
> #endif /* _LINUX_HUGETLB_VMEMMAP_H */
> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> index d3076a7a3783..60fc6cd6cd23 100644
> --- a/mm/sparse-vmemmap.c
> +++ b/mm/sparse-vmemmap.c
> @@ -40,7 +40,8 @@
> * @remap_pte: called for each lowest-level entry (PTE).
> * @reuse_page: the page which is reused for the tail vmemmap pages.
> * @reuse_addr: the virtual address of the @reuse_page page.
> - * @vmemmap_pages: the list head of the vmemmap pages that can be freed.
> + * @vmemmap_pages: the list head of the vmemmap pages that can be freed
> + * or is mapped from.
> */
> struct vmemmap_remap_walk {
> void (*remap_pte)(pte_t *pte, unsigned long addr,
> @@ -237,6 +238,78 @@ void vmemmap_remap_free(unsigned long start, unsigned long end,
> free_vmemmap_page_list(&vmemmap_pages);
> }
>
> +static void vmemmap_restore_pte(pte_t *pte, unsigned long addr,
> + struct vmemmap_remap_walk *walk)
> +{
> + pgprot_t pgprot = PAGE_KERNEL;
> + struct page *page;
> + void *to;
> +
> + BUG_ON(pte_page(*pte) != walk->reuse_page);
> +
> + page = list_first_entry(walk->vmemmap_pages, struct page, lru);
> + list_del(&page->lru);
> + to = page_to_virt(page);
> + copy_page(to, (void *)walk->reuse_addr);
> +
> + set_pte_at(&init_mm, addr, pte, mk_pte(page, pgprot));
> +}
> +
> +static int alloc_vmemmap_page_list(unsigned long start, unsigned long end,
> + gfp_t gfp_mask, struct list_head *list)
> +{
> + unsigned long nr_pages = (end - start) >> PAGE_SHIFT;
> + int nid = page_to_nid((struct page *)start);
> + struct page *page, *next;
> +
> + while (nr_pages--) {
> + page = alloc_pages_node(nid, gfp_mask, 0);
> + if (!page)
> + goto out;
> + list_add_tail(&page->lru, list);
> + }
> +
> + return 0;
> +out:
> + list_for_each_entry_safe(page, next, list, lru)
> + __free_pages(page, 0);
> + return -ENOMEM;
> +}
> +
> +/**
> + * vmemmap_remap_alloc - remap the vmemmap virtual address range [@start, end)
> + * to the page which is from the @vmemmap_pages
> + * respectively.
> + * @start: start address of the vmemmap virtual address range that we want
> + * to remap.
> + * @end: end address of the vmemmap virtual address range that we want to
> + * remap.
> + * @reuse: reuse address.
> + * @gpf_mask: GFP flag for allocating vmemmap pages.
> + */
> +int vmemmap_remap_alloc(unsigned long start, unsigned long end,
> + unsigned long reuse, gfp_t gfp_mask)
> +{
> + LIST_HEAD(vmemmap_pages);
> + struct vmemmap_remap_walk walk = {
> + .remap_pte = vmemmap_restore_pte,
> + .reuse_addr = reuse,
> + .vmemmap_pages = &vmemmap_pages,
> + };
> +
> + /* See the comment in the vmemmap_remap_free(). */
> + BUG_ON(start - reuse != PAGE_SIZE);
> +
> + might_sleep_if(gfpflags_allow_blocking(gfp_mask));
> +
> + if (alloc_vmemmap_page_list(start, end, gfp_mask, &vmemmap_pages))
> + return -ENOMEM;
> +
> + vmemmap_remap_range(reuse, end, &walk);
> +
> + return 0;
> +}
> +
> /*
> * Allocate a block of memory to be used to back the virtual memory map
> * or to back the page tables that are used to create the mapping.
>

2021-03-04 08:09:52

by Balbir Singh

Subject: Re: [PATCH v17 1/9] mm: memory_hotplug: factor out bootmem core functions to bootmem_info.c

On 26/2/21 12:21 am, Muchun Song wrote:
> Move bootmem info registration common API to individual bootmem_info.c.
> And we will use {get,put}_page_bootmem() to initialize the page for the
> vmemmap pages or free the vmemmap pages to buddy in the later patch.
> So move them out of CONFIG_MEMORY_HOTPLUG_SPARSE. This is just code
> movement without any functional change.
>
> Signed-off-by: Muchun Song <[email protected]>
> Acked-by: Mike Kravetz <[email protected]>
> Reviewed-by: Oscar Salvador <[email protected]>
> Reviewed-by: David Hildenbrand <[email protected]>
> Reviewed-by: Miaohe Lin <[email protected]>
...

> diff --git a/mm/bootmem_info.c b/mm/bootmem_info.c
> new file mode 100644
> index 000000000000..fcab5a3f8cc0
> --- /dev/null
> +++ b/mm/bootmem_info.c
> @@ -0,0 +1,124 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * linux/mm/bootmem_info.c
> + *
> + * Copyright (C)

Looks like incomplete


Balbir Singh

2021-03-04 08:17:25

by Muchun Song

Subject: Re: [External] Re: [PATCH v17 4/9] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page

On Wed, Mar 3, 2021 at 10:04 AM Mike Kravetz <[email protected]> wrote:
>
> On 2/25/21 5:21 AM, Muchun Song wrote:
> > When we free a HugeTLB page to the buddy allocator, we should allocate
> > the vmemmap pages associated with it. But we may cannot allocate vmemmap
> > pages when the system is under memory pressure, in this case, we just
> > refuse to free the HugeTLB page instead of looping forever trying to
> > allocate the pages. This changes some behavior (list below) on some
> > cassociated with it.orner cases.
>
> Suggest rewording this as:
>
> When we free a HugeTLB page to the buddy allocator, we need to allocate
> the vmemmap pages associated with it. However, we may not be able to
> allocate the vmemmap pages when the system is under memory pressure. In
> this case, we just refuse to free the HugeTLB page. This changes behavior
> in some corner cases as listed below:

Thanks Mike. I will use this.

>
> >
> > 1) Failing to free a huge page triggered by the user (decrease nr_pages).
> >
> > Need try again later by the user.
>
> User needs to try again later.
> >
> > 2) Failing to free a surplus huge page when freed by the application.
> >
> > Try again later when freeing a huge page next time.
> >
> > 3) Failing to dissolve a free huge page on ZONE_MOVABLE via
> > offline_pages().
> >
> > This is a bit unfortunate if we have plenty of ZONE_MOVABLE memory
> > but are low on kernel memory. For example, migration of huge pages
> > would still work, however, dissolving the free page does not work.
> > This is a corner cases. When the system is that much under memory
> > pressure, offlining/unplug can be expected to fail. This is
> > unfortunate because it prevents from the memory offlining which
> > shouldn't happen for movable zones. People depending on the memory
> > hotplug and movable zone should carefuly consider whether savings
> > on unmovable memory are worth losing their hotplug functionality
> > in some situations.
> >
>
> Possible wording change:
> This can happen when we have plenty of ZONE_MOVABLE memory, but
> not enough kernel memory to allocate vmemmmap pages. We may even
> be able to migrate huge page contents, but will not be able to
> dissolve the source huge page. This will prevent an offline
> operation and is unfortunate as memory offlining is expected to
> succeed on movable zones. Users that depend on memory hotplug
> to succeed for movable zones should carefully consider whether the
> memory savings gained from this feature are worth the risk of
> possibly not being able to offline memory in certain situations.

OK. Will use. Thanks.

>
> > 4) Failing to dissolve a huge page on CMA/ZONE_MOVABLE via
> > alloc_contig_range() - once we have that handling in place. Mainly
> > affects CMA and virtio-mem.
> >
> > Similar to 3). virito-mem will handle migration errors gracefully.
> > CMA might be able to fallback on other free areas within the CMA
> > region.
> >
> > Vmemmap pages are allocated from the page freeing context. In order for
> > those allocations to be not disruptive (e.g. trigger oom killer)
> > __GFP_NORETRY is used. hugetlb_lock is dropped for the allocation
> > because a non sleeping allocation would be too fragile and it could fail
> > too easily under memory pressure. GFP_ATOMIC or other modes to access
> > memory reserves is not used because we want to prevent consuming
> > reserves under heavy hugetlb freeing.
> >
> > Signed-off-by: Muchun Song <[email protected]>
> > ---
> > Documentation/admin-guide/mm/hugetlbpage.rst | 8 +++
> > include/linux/mm.h | 2 +
> > mm/hugetlb.c | 92 +++++++++++++++++++++-------
> > mm/hugetlb_vmemmap.c | 32 ++++++----
> > mm/hugetlb_vmemmap.h | 23 +++++++
> > mm/sparse-vmemmap.c | 75 ++++++++++++++++++++++-
> > 6 files changed, 197 insertions(+), 35 deletions(-)
> >
> > diff --git a/Documentation/admin-guide/mm/hugetlbpage.rst b/Documentation/admin-guide/mm/hugetlbpage.rst
> > index f7b1c7462991..6988895d09a8 100644
> > --- a/Documentation/admin-guide/mm/hugetlbpage.rst
> > +++ b/Documentation/admin-guide/mm/hugetlbpage.rst
> > @@ -60,6 +60,10 @@ HugePages_Surp
> > the pool above the value in ``/proc/sys/vm/nr_hugepages``. The
> > maximum number of surplus huge pages is controlled by
> > ``/proc/sys/vm/nr_overcommit_hugepages``.
> > + Note: When the feature of freeing unused vmemmap pages associated
> > + with each hugetlb page is enabled, the number of surplus huge pages
> > + may be temporarily larger than the maximum number of surplus huge
> > + pages when the system is under memory pressure.
> > Hugepagesize
> > is the default hugepage size (in Kb).
> > Hugetlb
> > @@ -80,6 +84,10 @@ returned to the huge page pool when freed by a task. A user with root
> > privileges can dynamically allocate more or free some persistent huge pages
> > by increasing or decreasing the value of ``nr_hugepages``.
> >
> > +Note: When the feature of freeing unused vmemmap pages associated with each
> > +hugetlb page is enabled, we can fail to free the huge pages triggered by
> > +the user when ths system is under memory pressure. Please try again later.
> > +
> > Pages that are used as huge pages are reserved inside the kernel and cannot
> > be used for other purposes. Huge pages cannot be swapped out under
> > memory pressure.
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 4ddfc31f21c6..77693c944a36 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -2973,6 +2973,8 @@ static inline void print_vma_addr(char *prefix, unsigned long rip)
> >
> > void vmemmap_remap_free(unsigned long start, unsigned long end,
> > unsigned long reuse);
> > +int vmemmap_remap_alloc(unsigned long start, unsigned long end,
> > + unsigned long reuse, gfp_t gfp_mask);
> >
> > void *sparse_buffer_alloc(unsigned long size);
> > struct page * __populate_section_memmap(unsigned long pfn,
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 43fed6785322..b6e4e3f31ad2 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -1304,16 +1304,59 @@ static inline void destroy_compound_gigantic_page(struct page *page,
> > unsigned int order) { }
> > #endif
> >
> > -static void update_and_free_page(struct hstate *h, struct page *page)
> > +static int update_and_free_page(struct hstate *h, struct page *page)
> > + __releases(&hugetlb_lock) __acquires(&hugetlb_lock)
> > {
> > int i;
> > struct page *subpage = page;
> > + int nid = page_to_nid(page);
> >
> > if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
> > - return;
> > + return 0;
> >
> > h->nr_huge_pages--;
> > - h->nr_huge_pages_node[page_to_nid(page)]--;
> > + h->nr_huge_pages_node[nid]--;
> > + VM_BUG_ON_PAGE(hugetlb_cgroup_from_page(page), page);
> > + VM_BUG_ON_PAGE(hugetlb_cgroup_from_page_rsvd(page), page);
> > + set_page_refcounted(page);
> > + set_compound_page_dtor(page, NULL_COMPOUND_DTOR);
> > +
> > + /*
> > + * If the vmemmap pages associated with the HugeTLB page can be
> > + * optimized or the page is gigantic, we might block in
> > + * alloc_huge_page_vmemmap() or free_gigantic_page(). In both
> > + * cases, drop the hugetlb_lock.
> > + */
> > + if (free_vmemmap_pages_per_hpage(h) || hstate_is_gigantic(h))
> > + spin_unlock(&hugetlb_lock);
> > +
> > + if (alloc_huge_page_vmemmap(h, page)) {
> > + spin_lock(&hugetlb_lock);
> > + INIT_LIST_HEAD(&page->lru);
> > + set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
> > + h->nr_huge_pages++;
> > + h->nr_huge_pages_node[nid]++;
> > +
> > + /*
> > + * If we cannot allocate vmemmap pages, just refuse to free the
> > + * page and put the page back on the hugetlb free list and treat
> > + * as a surplus page.
> > + */
> > + h->surplus_huge_pages++;
> > + h->surplus_huge_pages_node[nid]++;
> > +
> > + /*
> > + * The refcount can be perfectly increased by memory-failure or
>
> Reword The refcount can possibly be increased by memory-failure or
> soft_offline handlers.

Make sense.

>
> > + * soft_offline handlers.
> > + */
> > + if (likely(put_page_testzero(page))) {
> > + arch_clear_hugepage_flags(page);
> > + enqueue_huge_page(h, page);
> > + }
> > +
> > + return -ENOMEM;
> > + }
> > +
> > for (i = 0; i < pages_per_huge_page(h);
> > i++, subpage = mem_map_next(subpage, page, i)) {
> > subpage->flags &= ~(1 << PG_locked | 1 << PG_error |
> > @@ -1321,22 +1364,18 @@ static void update_and_free_page(struct hstate *h, struct page *page)
> > 1 << PG_active | 1 << PG_private |
> > 1 << PG_writeback);
> > }
> > - VM_BUG_ON_PAGE(hugetlb_cgroup_from_page(page), page);
> > - VM_BUG_ON_PAGE(hugetlb_cgroup_from_page_rsvd(page), page);
> > - set_compound_page_dtor(page, NULL_COMPOUND_DTOR);
> > - set_page_refcounted(page);
> > +
> > if (hstate_is_gigantic(h)) {
> > - /*
> > - * Temporarily drop the hugetlb_lock, because
> > - * we might block in free_gigantic_page().
> > - */
> > - spin_unlock(&hugetlb_lock);
> > destroy_compound_gigantic_page(page, huge_page_order(h));
> > free_gigantic_page(page, huge_page_order(h));
> > - spin_lock(&hugetlb_lock);
> > } else {
> > __free_pages(page, huge_page_order(h));
> > }
> > +
> > + if (free_vmemmap_pages_per_hpage(h) || hstate_is_gigantic(h))
> > + spin_lock(&hugetlb_lock);
> > +
> > + return 0;
> > }
> >
> > struct hstate *size_to_hstate(unsigned long size)
> > @@ -1404,9 +1443,9 @@ static void __free_huge_page(struct page *page)
> > } else if (h->surplus_huge_pages_node[nid]) {
> > /* remove the page from active list */
> > list_del(&page->lru);
> > - update_and_free_page(h, page);
> > h->surplus_huge_pages--;
> > h->surplus_huge_pages_node[nid]--;
> > + update_and_free_page(h, page);
> > } else {
> > arch_clear_hugepage_flags(page);
> > enqueue_huge_page(h, page);
> > @@ -1447,7 +1486,7 @@ void free_huge_page(struct page *page)
> > /*
> > * Defer freeing if in non-task context to avoid hugetlb_lock deadlock.
> > */
> > - if (!in_task()) {
> > + if (!in_atomic()) {
>
> That should be "if (in_atomic()) instead of "if (!in_atomic())"

Very thanks. It is my mistake. I will fix it.

>
> Do note that there is an ongoing discussion about calling free_huge_page
> in various contexts.
>
> https://lore.kernel.org/linux-mm/[email protected]/

Thanks for your reminder. I will take a closer look at what the problem is.

>
> This discussion/issue is independent of this patch. Since that issue
> deals with existing code, we will need to come up with a solution there
> first. A solution there may impact how free_huge_page is structured and
> may impact this patch.
>
> The in_atomic() check is insufficient to handle all cases. It is better
> than !in_task(), but still does not cover all cases.
>
> The rest of the patch looks good to me.
> --
> Mike Kravetz
>
> > /*
> > * Only call schedule_work() if hpage_freelist is previously
> > * empty. Otherwise, schedule_work() had been called but the
> > @@ -1699,8 +1738,7 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed,
> > h->surplus_huge_pages--;
> > h->surplus_huge_pages_node[node]--;
> > }
> > - update_and_free_page(h, page);
> > - ret = 1;
> > + ret = !update_and_free_page(h, page);
> > break;
> > }
> > }
> > @@ -1713,10 +1751,14 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed,
> > * nothing for in-use hugepages and non-hugepages.
> > * This function returns values like below:
> > *
> > - * -EBUSY: failed to dissolved free hugepages or the hugepage is in-use
> > - * (allocated or reserved.)
> > - * 0: successfully dissolved free hugepages or the page is not a
> > - * hugepage (considered as already dissolved)
> > + * -ENOMEM: failed to allocate vmemmap pages to free the freed hugepages
> > + * when the system is under memory pressure and the feature of
> > + * freeing unused vmemmap pages associated with each hugetlb page
> > + * is enabled.
> > + * -EBUSY: failed to dissolved free hugepages or the hugepage is in-use
> > + * (allocated or reserved.)
> > + * 0: successfully dissolved free hugepages or the page is not a
> > + * hugepage (considered as already dissolved)
> > */
> > int dissolve_free_huge_page(struct page *page)
> > {
> > @@ -1771,8 +1813,12 @@ int dissolve_free_huge_page(struct page *page)
> > h->free_huge_pages--;
> > h->free_huge_pages_node[nid]--;
> > h->max_huge_pages--;
> > - update_and_free_page(h, head);
> > - rc = 0;
> > + rc = update_and_free_page(h, head);
> > + if (rc) {
> > + h->surplus_huge_pages--;
> > + h->surplus_huge_pages_node[nid]--;
> > + h->max_huge_pages++;
> > + }
> > }
> > out:
> > spin_unlock(&hugetlb_lock);
> > diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> > index 0209b736e0b4..f7ab3d99250a 100644
> > --- a/mm/hugetlb_vmemmap.c
> > +++ b/mm/hugetlb_vmemmap.c
> > @@ -181,21 +181,31 @@
> > #define RESERVE_VMEMMAP_NR 2U
> > #define RESERVE_VMEMMAP_SIZE (RESERVE_VMEMMAP_NR << PAGE_SHIFT)
> >
> > -/*
> > - * How many vmemmap pages associated with a HugeTLB page that can be freed
> > - * to the buddy allocator.
> > - *
> > - * Todo: Returns zero for now, which means the feature is disabled. We will
> > - * enable it once all the infrastructure is there.
> > - */
> > -static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
> > +static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h)
> > {
> > - return 0;
> > + return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT;
> > }
> >
> > -static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h)
> > +int alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
> > {
> > - return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT;
> > + unsigned long vmemmap_addr = (unsigned long)head;
> > + unsigned long vmemmap_end, vmemmap_reuse;
> > +
> > + if (!free_vmemmap_pages_per_hpage(h))
> > + return 0;
> > +
> > + vmemmap_addr += RESERVE_VMEMMAP_SIZE;
> > + vmemmap_end = vmemmap_addr + free_vmemmap_pages_size_per_hpage(h);
> > + vmemmap_reuse = vmemmap_addr - PAGE_SIZE;
> > + /*
> > + * The pages which the vmemmap virtual address range [@vmemmap_addr,
> > + * @vmemmap_end) are mapped to are freed to the buddy allocator, and
> > + * the range is mapped to the page which @vmemmap_reuse is mapped to.
> > + * When a HugeTLB page is freed to the buddy allocator, previously
> > + * discarded vmemmap pages must be allocated and remapping.
> > + */
> > + return vmemmap_remap_alloc(vmemmap_addr, vmemmap_end, vmemmap_reuse,
> > + GFP_KERNEL | __GFP_NORETRY | __GFP_THISNODE);
> > }
> >
> > void free_huge_page_vmemmap(struct hstate *h, struct page *head)
> > diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
> > index 6923f03534d5..a37771b0b82a 100644
> > --- a/mm/hugetlb_vmemmap.h
> > +++ b/mm/hugetlb_vmemmap.h
> > @@ -11,10 +11,33 @@
> > #include <linux/hugetlb.h>
> >
> > #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> > +int alloc_huge_page_vmemmap(struct hstate *h, struct page *head);
> > void free_huge_page_vmemmap(struct hstate *h, struct page *head);
> > +
> > +/*
> > + * How many vmemmap pages associated with a HugeTLB page that can be freed
> > + * to the buddy allocator.
> > + *
> > + * Todo: Returns zero for now, which means the feature is disabled. We will
> > + * enable it once all the infrastructure is there.
> > + */
> > +static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
> > +{
> > + return 0;
> > +}
> > #else
> > +static inline int alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
> > +{
> > + return 0;
> > +}
> > +
> > static inline void free_huge_page_vmemmap(struct hstate *h, struct page *head)
> > {
> > }
> > +
> > +static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
> > +{
> > + return 0;
> > +}
> > #endif /* CONFIG_HUGETLB_PAGE_FREE_VMEMMAP */
> > #endif /* _LINUX_HUGETLB_VMEMMAP_H */
> > diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> > index d3076a7a3783..60fc6cd6cd23 100644
> > --- a/mm/sparse-vmemmap.c
> > +++ b/mm/sparse-vmemmap.c
> > @@ -40,7 +40,8 @@
> > * @remap_pte: called for each lowest-level entry (PTE).
> > * @reuse_page: the page which is reused for the tail vmemmap pages.
> > * @reuse_addr: the virtual address of the @reuse_page page.
> > - * @vmemmap_pages: the list head of the vmemmap pages that can be freed.
> > + * @vmemmap_pages: the list head of the vmemmap pages that can be freed
> > + * or is mapped from.
> > */
> > struct vmemmap_remap_walk {
> > void (*remap_pte)(pte_t *pte, unsigned long addr,
> > @@ -237,6 +238,78 @@ void vmemmap_remap_free(unsigned long start, unsigned long end,
> > free_vmemmap_page_list(&vmemmap_pages);
> > }
> >
> > +static void vmemmap_restore_pte(pte_t *pte, unsigned long addr,
> > + struct vmemmap_remap_walk *walk)
> > +{
> > + pgprot_t pgprot = PAGE_KERNEL;
> > + struct page *page;
> > + void *to;
> > +
> > + BUG_ON(pte_page(*pte) != walk->reuse_page);
> > +
> > + page = list_first_entry(walk->vmemmap_pages, struct page, lru);
> > + list_del(&page->lru);
> > + to = page_to_virt(page);
> > + copy_page(to, (void *)walk->reuse_addr);
> > +
> > + set_pte_at(&init_mm, addr, pte, mk_pte(page, pgprot));
> > +}
> > +
> > +static int alloc_vmemmap_page_list(unsigned long start, unsigned long end,
> > + gfp_t gfp_mask, struct list_head *list)
> > +{
> > + unsigned long nr_pages = (end - start) >> PAGE_SHIFT;
> > + int nid = page_to_nid((struct page *)start);
> > + struct page *page, *next;
> > +
> > + while (nr_pages--) {
> > + page = alloc_pages_node(nid, gfp_mask, 0);
> > + if (!page)
> > + goto out;
> > + list_add_tail(&page->lru, list);
> > + }
> > +
> > + return 0;
> > +out:
> > + list_for_each_entry_safe(page, next, list, lru)
> > + __free_pages(page, 0);
> > + return -ENOMEM;
> > +}
> > +
> > +/**
> > + * vmemmap_remap_alloc - remap the vmemmap virtual address range [@start, end)
> > + * to the page which is from the @vmemmap_pages
> > + * respectively.
> > + * @start: start address of the vmemmap virtual address range that we want
> > + * to remap.
> > + * @end: end address of the vmemmap virtual address range that we want to
> > + * remap.
> > + * @reuse: reuse address.
> > + * @gpf_mask: GFP flag for allocating vmemmap pages.
> > + */
> > +int vmemmap_remap_alloc(unsigned long start, unsigned long end,
> > + unsigned long reuse, gfp_t gfp_mask)
> > +{
> > + LIST_HEAD(vmemmap_pages);
> > + struct vmemmap_remap_walk walk = {
> > + .remap_pte = vmemmap_restore_pte,
> > + .reuse_addr = reuse,
> > + .vmemmap_pages = &vmemmap_pages,
> > + };
> > +
> > + /* See the comment in the vmemmap_remap_free(). */
> > + BUG_ON(start - reuse != PAGE_SIZE);
> > +
> > + might_sleep_if(gfpflags_allow_blocking(gfp_mask));
> > +
> > + if (alloc_vmemmap_page_list(start, end, gfp_mask, &vmemmap_pages))
> > + return -ENOMEM;
> > +
> > + vmemmap_remap_range(reuse, end, &walk);
> > +
> > + return 0;
> > +}
> > +
> > /*
> > * Allocate a block of memory to be used to back the virtual memory map
> > * or to back the page tables that are used to create the mapping.
> >

2021-03-04 14:38:50

by Balbir Singh

Subject: Re: [PATCH v17 0/9] Free some vmemmap pages of HugeTLB page

On 26/2/21 12:21 am, Muchun Song wrote:
> Hi all,
>
> This patch series will free some vmemmap pages(struct page structures)
> associated with each hugetlbpage when preallocated to save memory.
>
> In order to reduce the difficulty of the first version of code review.
> From this version, we disable PMD/huge page mapping of vmemmap if this
> feature was enabled. This accutualy eliminate a bunch of the complex code
> doing page table manipulation. When this patch series is solid, we cam add
> the code of vmemmap page table manipulation in the future.
>
> The struct page structures (page structs) are used to describe a physical
> page frame. By default, there is a one-to-one mapping from a page frame to
> it's corresponding page struct.
>
> The HugeTLB pages consist of multiple base page size pages and is supported
> by many architectures. See hugetlbpage.rst in the Documentation directory
> for more details. On the x86 architecture, HugeTLB pages of size 2MB and 1GB
> are currently supported. Since the base page size on x86 is 4KB, a 2MB
> HugeTLB page consists of 512 base pages and a 1GB HugeTLB page consists of
> 4096 base pages. For each base page, there is a corresponding page struct.
>
> Within the HugeTLB subsystem, only the first 4 page structs are used to
> contain unique information about a HugeTLB page. HUGETLB_CGROUP_MIN_ORDER
> provides this upper limit. The only 'useful' information in the remaining
> page structs is the compound_head field, and this field is the same for all
> tail pages.

The HUGETLB_CGROUP_MIN_ORDER is only when CGROUP_HUGETLB is enabled, but I guess
that does not matter

>
> By removing redundant page structs for HugeTLB pages, memory can returned to
> the buddy allocator for other uses.
>
> When the system boot up, every 2M HugeTLB has 512 struct page structs which
> size is 8 pages(sizeof(struct page) * 512 / PAGE_SIZE).
>
> HugeTLB struct pages(8 pages) page frame(8 pages)
> +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+
> | | | 0 | -------------> | 0 |
> | | +-----------+ +-----------+
> | | | 1 | -------------> | 1 |
> | | +-----------+ +-----------+
> | | | 2 | -------------> | 2 |
> | | +-----------+ +-----------+
> | | | 3 | -------------> | 3 |
> | | +-----------+ +-----------+
> | | | 4 | -------------> | 4 |
> | 2MB | +-----------+ +-----------+
> | | | 5 | -------------> | 5 |
> | | +-----------+ +-----------+
> | | | 6 | -------------> | 6 |
> | | +-----------+ +-----------+
> | | | 7 | -------------> | 7 |
> | | +-----------+ +-----------+
> | |
> | |
> | |
> +-----------+
>
> The value of page->compound_head is the same for all tail pages. The first
> page of page structs (page 0) associated with the HugeTLB page contains the 4
> page structs necessary to describe the HugeTLB. The only use of the remaining
> pages of page structs (page 1 to page 7) is to point to page->compound_head.
> Therefore, we can remap pages 2 to 7 to page 1. Only 2 pages of page structs
> will be used for each HugeTLB page. This will allow us to free the remaining
> 6 pages to the buddy allocator.

What is page 1 used for? page 0 carries the 4 struct pages needed, does compound_head
need a full page? IOW, why do we need two full pages -- may be the patches have the
answer to something I am missing?

>
> Here is how things look after remapping.
>
> HugeTLB struct pages(8 pages) page frame(8 pages)
> +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+
> | | | 0 | -------------> | 0 |
> | | +-----------+ +-----------+
> | | | 1 | -------------> | 1 |
> | | +-----------+ +-----------+
> | | | 2 | ----------------^ ^ ^ ^ ^ ^
> | | +-----------+ | | | | |
> | | | 3 | ------------------+ | | | |
> | | +-----------+ | | | |
> | | | 4 | --------------------+ | | |
> | 2MB | +-----------+ | | |
> | | | 5 | ----------------------+ | |
> | | +-----------+ | |
> | | | 6 | ------------------------+ |
> | | +-----------+ |
> | | | 7 | --------------------------+
> | | +-----------+
> | |
> | |
> | |
> +-----------+
>
> When a HugeTLB is freed to the buddy system, we should allocate 6 pages for
> vmemmap pages and restore the previous mapping relationship.
>

Can these 6 pages come from the hugeTLB page itself? When you say 6 pages,
I presume you mean 6 pages of PAGE_SIZE

> Apart from 2MB HugeTLB page, we also have 1GB HugeTLB page. It is similar
> to the 2MB HugeTLB page. We also can use this approach to free the vmemmap
> pages.
>
> In this case, for the 1GB HugeTLB page, we can save 4094 pages. This is a
> very substantial gain. On our server, run some SPDK/QEMU applications which
> will use 1024GB hugetlbpage. With this feature enabled, we can save ~16GB
> (1G hugepage)/~12GB (2MB hugepage) memory.

Thanks,
Balbir Singh

2021-03-04 14:44:24

by Balbir Singh

Subject: Re: [PATCH v17 1/9] mm: memory_hotplug: factor out bootmem core functions to bootmem_info.c

On Wed, Mar 03, 2021 at 01:45:00PM +1100, Singh, Balbir wrote:
> On 26/2/21 12:21 am, Muchun Song wrote:
> > Move bootmem info registration common API to individual bootmem_info.c.
> > And we will use {get,put}_page_bootmem() to initialize the page for the
> > vmemmap pages or free the vmemmap pages to buddy in the later patch.
> > So move them out of CONFIG_MEMORY_HOTPLUG_SPARSE. This is just code
> > movement without any functional change.
> >
> > Signed-off-by: Muchun Song <[email protected]>
> > Acked-by: Mike Kravetz <[email protected]>
> > Reviewed-by: Oscar Salvador <[email protected]>
> > Reviewed-by: David Hildenbrand <[email protected]>
> > Reviewed-by: Miaohe Lin <[email protected]>
> ...
>
> > diff --git a/mm/bootmem_info.c b/mm/bootmem_info.c
> > new file mode 100644
> > index 000000000000..fcab5a3f8cc0
> > --- /dev/null
> > +++ b/mm/bootmem_info.c
> > @@ -0,0 +1,124 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/*
> > + * linux/mm/bootmem_info.c
> > + *
> > + * Copyright (C)
>
> Looks like incomplete
>
Not that my comment was, I should have said

The copyright looks very incomplete

Balbir Singh.

2021-03-04 14:55:07

by Balbir Singh

Subject: Re: [PATCH v17 2/9] mm: hugetlb: introduce a new config HUGETLB_PAGE_FREE_VMEMMAP

On Thu, Feb 25, 2021 at 09:21:23PM +0800, Muchun Song wrote:
> The option HUGETLB_PAGE_FREE_VMEMMAP allows for the freeing of
> some vmemmap pages associated with pre-allocated HugeTLB pages.
> For example, on X86_64 6 vmemmap pages of size 4KB each can be
> saved for each 2MB HugeTLB page. 4094 vmemmap pages of size 4KB
> each can be saved for each 1GB HugeTLB page.
>
> When a HugeTLB page is allocated or freed, the vmemmap array
> representing the range associated with the page will need to be
> remapped. When a page is allocated, vmemmap pages are freed
> after remapping. When a page is freed, previously discarded
> vmemmap pages must be allocated before remapping.
>
> The config option is introduced early so that supporting code
> can be written to depend on the option. The initial version of
> the code only provides support for x86-64.
>
> Like other code which frees vmemmap, this config option depends on
> HAVE_BOOTMEM_INFO_NODE. The routine register_page_bootmem_info() is
> used to register bootmem info. Therefore, make sure
> register_page_bootmem_info is enabled if HUGETLB_PAGE_FREE_VMEMMAP
> is defined.
>
> Signed-off-by: Muchun Song <[email protected]>
> Reviewed-by: Oscar Salvador <[email protected]>
> Acked-by: Mike Kravetz <[email protected]>
> Reviewed-by: Miaohe Lin <[email protected]>
> ---

Reviewed-by: Balbir Singh <[email protected]>

2021-03-05 00:01:53

by Muchun Song

Subject: Re: [External] Re: [PATCH v17 0/9] Free some vmemmap pages of HugeTLB page

On Thu, Mar 4, 2021 at 11:14 AM Singh, Balbir <[email protected]> wrote:
>
> On 26/2/21 12:21 am, Muchun Song wrote:
> > Hi all,
> >
> > This patch series will free some vmemmap pages(struct page structures)
> > associated with each hugetlbpage when preallocated to save memory.
> >
> > In order to reduce the difficulty of the first version of code review.
> > From this version, we disable PMD/huge page mapping of vmemmap if this
> > feature was enabled. This accutualy eliminate a bunch of the complex code
> > doing page table manipulation. When this patch series is solid, we cam add
> > the code of vmemmap page table manipulation in the future.
> >
> > The struct page structures (page structs) are used to describe a physical
> > page frame. By default, there is a one-to-one mapping from a page frame to
> > it's corresponding page struct.
> >
> > The HugeTLB pages consist of multiple base page size pages and is supported
> > by many architectures. See hugetlbpage.rst in the Documentation directory
> > for more details. On the x86 architecture, HugeTLB pages of size 2MB and 1GB
> > are currently supported. Since the base page size on x86 is 4KB, a 2MB
> > HugeTLB page consists of 512 base pages and a 1GB HugeTLB page consists of
> > 4096 base pages. For each base page, there is a corresponding page struct.
> >
> > Within the HugeTLB subsystem, only the first 4 page structs are used to
> > contain unique information about a HugeTLB page. HUGETLB_CGROUP_MIN_ORDER
> > provides this upper limit. The only 'useful' information in the remaining
> > page structs is the compound_head field, and this field is the same for all
> > tail pages.
>
> The HUGETLB_CGROUP_MIN_ORDER is only when CGROUP_HUGETLB is enabled, but I guess
> that does not matter

Agree.

>
> >
> > By removing redundant page structs for HugeTLB pages, memory can returned to
> > the buddy allocator for other uses.
> >
> > When the system boot up, every 2M HugeTLB has 512 struct page structs which
> > size is 8 pages(sizeof(struct page) * 512 / PAGE_SIZE).
> >
> > HugeTLB struct pages(8 pages) page frame(8 pages)
> > +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+
> > | | | 0 | -------------> | 0 |
> > | | +-----------+ +-----------+
> > | | | 1 | -------------> | 1 |
> > | | +-----------+ +-----------+
> > | | | 2 | -------------> | 2 |
> > | | +-----------+ +-----------+
> > | | | 3 | -------------> | 3 |
> > | | +-----------+ +-----------+
> > | | | 4 | -------------> | 4 |
> > | 2MB | +-----------+ +-----------+
> > | | | 5 | -------------> | 5 |
> > | | +-----------+ +-----------+
> > | | | 6 | -------------> | 6 |
> > | | +-----------+ +-----------+
> > | | | 7 | -------------> | 7 |
> > | | +-----------+ +-----------+
> > | |
> > | |
> > | |
> > +-----------+
> >
> > The value of page->compound_head is the same for all tail pages. The first
> > page of page structs (page 0) associated with the HugeTLB page contains the 4
> > page structs necessary to describe the HugeTLB. The only use of the remaining
> > pages of page structs (page 1 to page 7) is to point to page->compound_head.
> > Therefore, we can remap pages 2 to 7 to page 1. Only 2 pages of page structs
> > will be used for each HugeTLB page. This will allow us to free the remaining
> > 6 pages to the buddy allocator.
>
> What is page 1 used for? page 0 carries the 4 struct pages needed, does compound_head
> need a full page? IOW, why do we need two full pages -- may be the patches have the
> answer to something I am missing?

Yeah. It really can free 7 pages. But we need some work to support this. Why?

Now for the 2MB HugeTLB page, we only free 6 vmemmap pages. we really can
free 7 vmemmap pages. In this case, we can see 8 of the 512 struct page
structures have been set PG_head flag. If we can adjust compound_head()
slightly and make compound_head() return the real head struct page when
the parameter is the tail struct page but with PG_head flag set.

In order to make the code evolution route clearer. This feature can be
a separate patch (and send it out) after this patchset is solid and applied.

>
> >
> > Here is how things look after remapping.
> >
> > HugeTLB struct pages(8 pages) page frame(8 pages)
> > +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+
> > | | | 0 | -------------> | 0 |
> > | | +-----------+ +-----------+
> > | | | 1 | -------------> | 1 |
> > | | +-----------+ +-----------+
> > | | | 2 | ----------------^ ^ ^ ^ ^ ^
> > | | +-----------+ | | | | |
> > | | | 3 | ------------------+ | | | |
> > | | +-----------+ | | | |
> > | | | 4 | --------------------+ | | |
> > | 2MB | +-----------+ | | |
> > | | | 5 | ----------------------+ | |
> > | | +-----------+ | |
> > | | | 6 | ------------------------+ |
> > | | +-----------+ |
> > | | | 7 | --------------------------+
> > | | +-----------+
> > | |
> > | |
> > | |
> > +-----------+
> >
> > When a HugeTLB is freed to the buddy system, we should allocate 6 pages for
> > vmemmap pages and restore the previous mapping relationship.
> >
>
> Can these 6 pages come from the hugeTLB page itself? When you say 6 pages,
> I presume you mean 6 pages of PAGE_SIZE

There was a decent discussion about this in a previous version of the
series starting here:

https://lore.kernel.org/linux-mm/20210126092942.GA10602@linux/

In this thread various other options were suggested and discussed.

Thanks.

>
> > Apart from 2MB HugeTLB page, we also have 1GB HugeTLB page. It is similar
> > to the 2MB HugeTLB page. We also can use this approach to free the vmemmap
> > pages.
> >
> > In this case, for the 1GB HugeTLB page, we can save 4094 pages. This is a
> > very substantial gain. On our server, run some SPDK/QEMU applications which
> > will use 1024GB hugetlbpage. With this feature enabled, we can save ~16GB
> > (1G hugepage)/~12GB (2MB hugepage) memory.
>
> Thanks,
> Balbir Singh
>
>
>
>
>
>
>
>
>
>
>
>
>

2021-03-05 00:03:30

by Muchun Song

Subject: Re: [External] Re: [PATCH v17 1/9] mm: memory_hotplug: factor out bootmem core functions to bootmem_info.c

On Thu, Mar 4, 2021 at 12:26 PM Balbir Singh <[email protected]> wrote:
>
> On Wed, Mar 03, 2021 at 01:45:00PM +1100, Singh, Balbir wrote:
> > On 26/2/21 12:21 am, Muchun Song wrote:
> > > Move bootmem info registration common API to individual bootmem_info.c.
> > > And we will use {get,put}_page_bootmem() to initialize the page for the
> > > vmemmap pages or free the vmemmap pages to buddy in the later patch.
> > > So move them out of CONFIG_MEMORY_HOTPLUG_SPARSE. This is just code
> > > movement without any functional change.
> > >
> > > Signed-off-by: Muchun Song <[email protected]>
> > > Acked-by: Mike Kravetz <[email protected]>
> > > Reviewed-by: Oscar Salvador <[email protected]>
> > > Reviewed-by: David Hildenbrand <[email protected]>
> > > Reviewed-by: Miaohe Lin <[email protected]>
> > ...
> >
> > > diff --git a/mm/bootmem_info.c b/mm/bootmem_info.c
> > > new file mode 100644
> > > index 000000000000..fcab5a3f8cc0
> > > --- /dev/null
> > > +++ b/mm/bootmem_info.c
> > > @@ -0,0 +1,124 @@
> > > +// SPDX-License-Identifier: GPL-2.0
> > > +/*
> > > + * linux/mm/bootmem_info.c
> > > + *
> > > + * Copyright (C)
> >
> > Looks like incomplete
> >
> Not that my comment was, I should have said
>
> The copyright looks very incomplete

Yes. Just copied from mm/memory_hotplug.c.
I can improve it in the next version. Thanks.

>
> Balbir Singh.

2021-03-05 01:03:26

by Balbir Singh

Subject: Re: [PATCH v17 3/9] mm: hugetlb: free the vmemmap pages associated with each HugeTLB page

On 26/2/21 12:21 am, Muchun Song wrote:
> Every HugeTLB has more than one struct page structure. We __know__ that
> we only use the first 4(HUGETLB_CGROUP_MIN_ORDER) struct page structures
> to store metadata associated with each HugeTLB.
>
> There are a lot of struct page structures associated with each HugeTLB
> page. For tail pages, the value of compound_head is the same. So we can
> reuse first page of tail page structures. We map the virtual addresses
> of the remaining pages of tail page structures to the first tail page
> struct, and then free these page frames. Therefore, we need to reserve
> two pages as vmemmap areas.
>
> When we allocate a HugeTLB page from the buddy, we can free some vmemmap
> pages associated with each HugeTLB page. It is more appropriate to do it
> in the prep_new_huge_page().
>
> The free_vmemmap_pages_per_hpage(), which indicates how many vmemmap
> pages associated with a HugeTLB page can be freed, returns zero for
> now, which means the feature is disabled. We will enable it once all
> the infrastructure is there.
>
> Signed-off-by: Muchun Song <[email protected]>
> Reviewed-by: Oscar Salvador <[email protected]>
> ---
> include/linux/bootmem_info.h | 27 +++++-
> include/linux/mm.h | 3 +
> mm/Makefile | 1 +
> mm/hugetlb.c | 3 +
> mm/hugetlb_vmemmap.c | 219 +++++++++++++++++++++++++++++++++++++++++++
> mm/hugetlb_vmemmap.h | 20 ++++
> mm/sparse-vmemmap.c | 207 ++++++++++++++++++++++++++++++++++++++++
> 7 files changed, 479 insertions(+), 1 deletion(-)
> create mode 100644 mm/hugetlb_vmemmap.c
> create mode 100644 mm/hugetlb_vmemmap.h
>
> diff --git a/include/linux/bootmem_info.h b/include/linux/bootmem_info.h
> index 4ed6dee1adc9..ec03a624dfa2 100644
> --- a/include/linux/bootmem_info.h
> +++ b/include/linux/bootmem_info.h
> @@ -2,7 +2,7 @@
> #ifndef __LINUX_BOOTMEM_INFO_H
> #define __LINUX_BOOTMEM_INFO_H
>
> -#include <linux/mmzone.h>
> +#include <linux/mm.h>
>
> /*
> * Types for free bootmem stored in page->lru.next. These have to be in
> @@ -22,6 +22,27 @@ void __init register_page_bootmem_info_node(struct pglist_data *pgdat);
> void get_page_bootmem(unsigned long info, struct page *page,
> unsigned long type);
> void put_page_bootmem(struct page *page);
> +
> +/*
> + * Any memory allocated via the memblock allocator and not via the
> + * buddy will be marked reserved already in the memmap. For those
> + * pages, we can call this function to free it to buddy allocator.
> + */
> +static inline void free_bootmem_page(struct page *page)
> +{
> + unsigned long magic = (unsigned long)page->freelist;
> +
> + /*
> + * The reserve_bootmem_region sets the reserved flag on bootmem
> + * pages.
> + */
> + VM_BUG_ON_PAGE(page_ref_count(page) != 2, page);
> +
> + if (magic == SECTION_INFO || magic == MIX_SECTION_INFO)
> + put_page_bootmem(page);
> + else
> + VM_BUG_ON_PAGE(1, page);
> +}
> #else
> static inline void register_page_bootmem_info_node(struct pglist_data *pgdat)
> {
> @@ -35,6 +56,10 @@ static inline void get_page_bootmem(unsigned long info, struct page *page,
> unsigned long type)
> {
> }
> +
> +static inline void free_bootmem_page(struct page *page)
> +{
> +}
> #endif
>
> #endif /* __LINUX_BOOTMEM_INFO_H */
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 77e64e3eac80..4ddfc31f21c6 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2971,6 +2971,9 @@ static inline void print_vma_addr(char *prefix, unsigned long rip)
> }
> #endif
>
> +void vmemmap_remap_free(unsigned long start, unsigned long end,
> + unsigned long reuse);
> +
> void *sparse_buffer_alloc(unsigned long size);
> struct page * __populate_section_memmap(unsigned long pfn,
> unsigned long nr_pages, int nid, struct vmem_altmap *altmap);
> diff --git a/mm/Makefile b/mm/Makefile
> index daabf86d7da8..3d7d57e3b55b 100644
> --- a/mm/Makefile
> +++ b/mm/Makefile
> @@ -71,6 +71,7 @@ obj-$(CONFIG_FRONTSWAP) += frontswap.o
> obj-$(CONFIG_ZSWAP) += zswap.o
> obj-$(CONFIG_HAS_DMA) += dmapool.o
> obj-$(CONFIG_HUGETLBFS) += hugetlb.o
> +obj-$(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP) += hugetlb_vmemmap.o
> obj-$(CONFIG_NUMA) += mempolicy.o
> obj-$(CONFIG_SPARSEMEM) += sparse.o
> obj-$(CONFIG_SPARSEMEM_VMEMMAP) += sparse-vmemmap.o
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index c232cb67dda2..43fed6785322 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -42,6 +42,7 @@
> #include <linux/userfaultfd_k.h>
> #include <linux/page_owner.h>
> #include "internal.h"
> +#include "hugetlb_vmemmap.h"
>
> int hugetlb_max_hstate __read_mostly;
> unsigned int default_hstate_idx;
> @@ -1463,6 +1464,8 @@ void free_huge_page(struct page *page)
>
> static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
> {
> + free_huge_page_vmemmap(h, page);
> +
> INIT_LIST_HEAD(&page->lru);
> set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
> set_hugetlb_cgroup(page, NULL);
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> new file mode 100644
> index 000000000000..0209b736e0b4
> --- /dev/null
> +++ b/mm/hugetlb_vmemmap.c
> @@ -0,0 +1,219 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Free some vmemmap pages of HugeTLB
> + *
> + * Copyright (c) 2020, Bytedance. All rights reserved.
> + *
> + * Author: Muchun Song <[email protected]>
> + *
> + * The struct page structures (page structs) are used to describe a physical
> + * page frame. By default, there is a one-to-one mapping from a page frame to
> + * it's corresponding page struct.
> + *
> + * HugeTLB pages consist of multiple base page size pages and is supported by
> + * many architectures. See hugetlbpage.rst in the Documentation directory for
> + * more details. On the x86-64 architecture, HugeTLB pages of size 2MB and 1GB
> + * are currently supported. Since the base page size on x86 is 4KB, a 2MB
> + * HugeTLB page consists of 512 base pages and a 1GB HugeTLB page consists of
> + * 4096 base pages. For each base page, there is a corresponding page struct.
> + *
> + * Within the HugeTLB subsystem, only the first 4 page structs are used to
> + * contain unique information about a HugeTLB page. HUGETLB_CGROUP_MIN_ORDER
> + * provides this upper limit. The only 'useful' information in the remaining
> + * page structs is the compound_head field, and this field is the same for all
> + * tail pages.
> + *
> + * By removing redundant page structs for HugeTLB pages, memory can be returned
> + * to the buddy allocator for other uses.
> + *
> + * Different architectures support different HugeTLB pages. For example, the
> + * following table is the HugeTLB page size supported by x86 and arm64
> + * architectures. Because arm64 supports 4k, 16k, and 64k base pages and
> + * supports contiguous entries, so it supports many kinds of sizes of HugeTLB
> + * page.
> + *
> + * +--------------+-----------+-----------------------------------------------+
> + * | Architecture | Page Size | HugeTLB Page Size |
> + * +--------------+-----------+-----------+-----------+-----------+-----------+
> + * | x86-64 | 4KB | 2MB | 1GB | | |
> + * +--------------+-----------+-----------+-----------+-----------+-----------+
> + * | | 4KB | 64KB | 2MB | 32MB | 1GB |
> + * | +-----------+-----------+-----------+-----------+-----------+
> + * | arm64 | 16KB | 2MB | 32MB | 1GB | |
> + * | +-----------+-----------+-----------+-----------+-----------+
> + * | | 64KB | 2MB | 512MB | 16GB | |
> + * +--------------+-----------+-----------+-----------+-----------+-----------+
> + *
> + * When the system boot up, every HugeTLB page has more than one struct page
> + * structs which size is (unit: pages):
> + *
> + * struct_size = HugeTLB_Size / PAGE_SIZE * sizeof(struct page) / PAGE_SIZE
> + *
> + * Where HugeTLB_Size is the size of the HugeTLB page. We know that the size
> + * of the HugeTLB page is always n times PAGE_SIZE. So we can get the following
> + * relationship.
> + *
> + * HugeTLB_Size = n * PAGE_SIZE
> + *
> + * Then,
> + *
> + * struct_size = n * PAGE_SIZE / PAGE_SIZE * sizeof(struct page) / PAGE_SIZE
> + * = n * sizeof(struct page) / PAGE_SIZE
> + *
> + * We can use huge mapping at the pud/pmd level for the HugeTLB page.
> + *
> + * For the HugeTLB page of the pmd level mapping, then
> + *
> + * struct_size = n * sizeof(struct page) / PAGE_SIZE
> + * = PAGE_SIZE / sizeof(pte_t) * sizeof(struct page) / PAGE_SIZE
> + * = sizeof(struct page) / sizeof(pte_t)
> + * = 64 / 8
> + * = 8 (pages)
> + *
> + * Where n is how many pte entries which one page can contains. So the value of
> + * n is (PAGE_SIZE / sizeof(pte_t)).
> + *
> + * This optimization only supports 64-bit system, so the value of sizeof(pte_t)
> + * is 8. And this optimization also applicable only when the size of struct page
> + * is a power of two. In most cases, the size of struct page is 64 bytes (e.g.
> + * x86-64 and arm64). So if we use pmd level mapping for a HugeTLB page, the
> + * size of struct page structs of it is 8 page frames which size depends on the
> + * size of the base page.
> + *
> + * For the HugeTLB page of the pud level mapping, then
> + *
> + * struct_size = PAGE_SIZE / sizeof(pmd_t) * struct_size(pmd)
> + * = PAGE_SIZE / 8 * 8 (pages)
> + * = PAGE_SIZE (pages)
> + *
> + * Where the struct_size(pmd) is the size of the struct page structs of a
> + * HugeTLB page of the pmd level mapping.
> + *
> + * E.g.: A 2MB HugeTLB page on x86_64 consists in 8 page frames while 1GB
> + * HugeTLB page consists in 4096.
> + *
> + * Next, we take the pmd level mapping of the HugeTLB page as an example to
> + * show the internal implementation of this optimization. There are 8 pages
> + * struct page structs associated with a HugeTLB page which is pmd mapped.
> + *
> + * Here is how things look before optimization.
> + *
> + * HugeTLB struct pages(8 pages) page frame(8 pages)
> + * +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+
> + * | | | 0 | -------------> | 0 |
> + * | | +-----------+ +-----------+
> + * | | | 1 | -------------> | 1 |
> + * | | +-----------+ +-----------+
> + * | | | 2 | -------------> | 2 |
> + * | | +-----------+ +-----------+
> + * | | | 3 | -------------> | 3 |
> + * | | +-----------+ +-----------+
> + * | | | 4 | -------------> | 4 |
> + * | PMD | +-----------+ +-----------+
> + * | level | | 5 | -------------> | 5 |
> + * | mapping | +-----------+ +-----------+
> + * | | | 6 | -------------> | 6 |
> + * | | +-----------+ +-----------+
> + * | | | 7 | -------------> | 7 |
> + * | | +-----------+ +-----------+
> + * | |
> + * | |
> + * | |
> + * +-----------+
> + *
> + * The value of page->compound_head is the same for all tail pages. The first
> + * page of page structs (page 0) associated with the HugeTLB page contains the 4
> + * page structs necessary to describe the HugeTLB. The only use of the remaining
> + * pages of page structs (page 1 to page 7) is to point to page->compound_head.
> + * Therefore, we can remap pages 2 to 7 to page 1. Only 2 pages of page structs
> + * will be used for each HugeTLB page. This will allow us to free the remaining
> + * 6 pages to the buddy allocator.
> + *
> + * Here is how things look after remapping.
> + *
> + * HugeTLB struct pages(8 pages) page frame(8 pages)
> + * +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+
> + * | | | 0 | -------------> | 0 |
> + * | | +-----------+ +-----------+
> + * | | | 1 | -------------> | 1 |
> + * | | +-----------+ +-----------+
> + * | | | 2 | ----------------^ ^ ^ ^ ^ ^
> + * | | +-----------+ | | | | |
> + * | | | 3 | ------------------+ | | | |
> + * | | +-----------+ | | | |
> + * | | | 4 | --------------------+ | | |
> + * | PMD | +-----------+ | | |
> + * | level | | 5 | ----------------------+ | |
> + * | mapping | +-----------+ | |
> + * | | | 6 | ------------------------+ |
> + * | | +-----------+ |
> + * | | | 7 | --------------------------+
> + * | | +-----------+
> + * | |
> + * | |
> + * | |
> + * +-----------+
> + *
> + * When a HugeTLB is freed to the buddy system, we should allocate 6 pages for
> + * vmemmap pages and restore the previous mapping relationship.
> + *
> + * For the HugeTLB page of the pud level mapping. It is similar to the former.
> + * We also can use this approach to free (PAGE_SIZE - 2) vmemmap pages.
> + *
> + * Apart from the HugeTLB page of the pmd/pud level mapping, some architectures
> + * (e.g. aarch64) provides a contiguous bit in the translation table entries
> + * that hints to the MMU to indicate that it is one of a contiguous set of
> + * entries that can be cached in a single TLB entry.
> + *
> + * The contiguous bit is used to increase the mapping size at the pmd and pte
> + * (last) level. So this type of HugeTLB page can be optimized only when its
> + * size of the struct page structs is greater than 2 pages.
> + */
> +#include "hugetlb_vmemmap.h"
> +
> +/*
> + * There are a lot of struct page structures associated with each HugeTLB page.
> + * For tail pages, the value of compound_head is the same. So we can reuse first
> + * page of tail page structures. We map the virtual addresses of the remaining
> + * pages of tail page structures to the first tail page struct, and then free
> + * these page frames. Therefore, we need to reserve two pages as vmemmap areas.
> + */
> +#define RESERVE_VMEMMAP_NR 2U
> +#define RESERVE_VMEMMAP_SIZE (RESERVE_VMEMMAP_NR << PAGE_SHIFT)
> +
> +/*
> + * How many vmemmap pages associated with a HugeTLB page that can be freed
> + * to the buddy allocator.
> + *
> + * Todo: Returns zero for now, which means the feature is disabled. We will
> + * enable it once all the infrastructure is there.
> + */
> +static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
> +{
> + return 0;
> +}
> +
> +static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h)
> +{
> + return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT;
> +}
> +
> +void free_huge_page_vmemmap(struct hstate *h, struct page *head)
> +{
> + unsigned long vmemmap_addr = (unsigned long)head;
> + unsigned long vmemmap_end, vmemmap_reuse;
> +
> + if (!free_vmemmap_pages_per_hpage(h))
> + return;
> +
> + vmemmap_addr += RESERVE_VMEMMAP_SIZE;
> + vmemmap_end = vmemmap_addr + free_vmemmap_pages_size_per_hpage(h);
> + vmemmap_reuse = vmemmap_addr - PAGE_SIZE;
> +
> + /*
> + * Remap the vmemmap virtual address range [@vmemmap_addr, @vmemmap_end)
> + * to the page which @vmemmap_reuse is mapped to, then free the pages
> + * which the range [@vmemmap_addr, @vmemmap_end] is mapped to.
> + */
> + vmemmap_remap_free(vmemmap_addr, vmemmap_end, vmemmap_reuse);
> +}
> diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
> new file mode 100644
> index 000000000000..6923f03534d5
> --- /dev/null
> +++ b/mm/hugetlb_vmemmap.h
> @@ -0,0 +1,20 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Free some vmemmap pages of HugeTLB
> + *
> + * Copyright (c) 2020, Bytedance. All rights reserved.
> + *
> + * Author: Muchun Song <[email protected]>
> + */
> +#ifndef _LINUX_HUGETLB_VMEMMAP_H
> +#define _LINUX_HUGETLB_VMEMMAP_H
> +#include <linux/hugetlb.h>
> +
> +#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> +void free_huge_page_vmemmap(struct hstate *h, struct page *head);
> +#else
> +static inline void free_huge_page_vmemmap(struct hstate *h, struct page *head)
> +{
> +}
> +#endif /* CONFIG_HUGETLB_PAGE_FREE_VMEMMAP */
> +#endif /* _LINUX_HUGETLB_VMEMMAP_H */
> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> index 16183d85a7d5..d3076a7a3783 100644
> --- a/mm/sparse-vmemmap.c
> +++ b/mm/sparse-vmemmap.c
> @@ -27,8 +27,215 @@
> #include <linux/spinlock.h>
> #include <linux/vmalloc.h>
> #include <linux/sched.h>
> +#include <linux/pgtable.h>
> +#include <linux/bootmem_info.h>
> +
> #include <asm/dma.h>
> #include <asm/pgalloc.h>
> +#include <asm/tlbflush.h>
> +
> +/**
> + * vmemmap_remap_walk - walk vmemmap page table
> + *
> + * @remap_pte: called for each lowest-level entry (PTE).
> + * @reuse_page: the page which is reused for the tail vmemmap pages.
> + * @reuse_addr: the virtual address of the @reuse_page page.
> + * @vmemmap_pages: the list head of the vmemmap pages that can be freed.
> + */
> +struct vmemmap_remap_walk {
> + void (*remap_pte)(pte_t *pte, unsigned long addr,
> + struct vmemmap_remap_walk *walk);
> + struct page *reuse_page;
> + unsigned long reuse_addr;
> + struct list_head *vmemmap_pages;
> +};
> +
> +static void vmemmap_pte_range(pmd_t *pmd, unsigned long addr,
> + unsigned long end,
> + struct vmemmap_remap_walk *walk)
> +{
> + pte_t *pte;
> +
> + pte = pte_offset_kernel(pmd, addr);
> +
> + /*
> + * The reuse_page is found 'first' in table walk before we start
> + * remapping (which is calling @walk->remap_pte).
> + */
> + if (!walk->reuse_page) {
> + BUG_ON(pte_none(*pte));
> + BUG_ON(walk->reuse_addr != addr);
> +
> + walk->reuse_page = pte_page(*pte++);

The concurrency semantics of this code are not clear, do we need READ_ONCE()/
WRITE_ONCE() semantics if this page walk is lockless? Can we run this code
in parallel on the same section? I presume not

> + /*
> + * Because the reuse address is part of the range that we are
> + * walking, skip the reuse address range.
> + */
> + addr += PAGE_SIZE;
> + }
> +
> + for (; addr != end; addr += PAGE_SIZE, pte++) {
> + BUG_ON(pte_none(*pte));
> +
> + walk->remap_pte(pte, addr, walk);
> + }
> +}
> +
> +static void vmemmap_pmd_range(pud_t *pud, unsigned long addr,
> + unsigned long end,
> + struct vmemmap_remap_walk *walk)
> +{
> + pmd_t *pmd;
> + unsigned long next;
> +
> + pmd = pmd_offset(pud, addr);
> + do {
> + BUG_ON(pmd_none(*pmd) || pmd_leaf(*pmd));
> +
> + next = pmd_addr_end(addr, end);
> + vmemmap_pte_range(pmd, addr, next, walk);
> + } while (pmd++, addr = next, addr != end);
> +}
> +
> +static void vmemmap_pud_range(p4d_t *p4d, unsigned long addr,
> + unsigned long end,
> + struct vmemmap_remap_walk *walk)
> +{
> + pud_t *pud;
> + unsigned long next;
> +
> + pud = pud_offset(p4d, addr);
> + do {
> + BUG_ON(pud_none(*pud));
> +
> + next = pud_addr_end(addr, end);
> + vmemmap_pmd_range(pud, addr, next, walk);
> + } while (pud++, addr = next, addr != end);
> +}
> +
> +static void vmemmap_p4d_range(pgd_t *pgd, unsigned long addr,
> + unsigned long end,
> + struct vmemmap_remap_walk *walk)
> +{
> + p4d_t *p4d;
> + unsigned long next;
> +
> + p4d = p4d_offset(pgd, addr);
> + do {
> + BUG_ON(p4d_none(*p4d));
> +
> + next = p4d_addr_end(addr, end);
> + vmemmap_pud_range(p4d, addr, next, walk);
> + } while (p4d++, addr = next, addr != end);
> +}
> +
> +static void vmemmap_remap_range(unsigned long start, unsigned long end,
> + struct vmemmap_remap_walk *walk)
> +{
> + unsigned long addr = start;
> + unsigned long next;
> + pgd_t *pgd;
> +
> + VM_BUG_ON(!IS_ALIGNED(start, PAGE_SIZE));
> + VM_BUG_ON(!IS_ALIGNED(end, PAGE_SIZE));
> +
> + pgd = pgd_offset_k(addr);
> + do {
> + BUG_ON(pgd_none(*pgd));
> +
> + next = pgd_addr_end(addr, end);
> + vmemmap_p4d_range(pgd, addr, next, walk);
> + } while (pgd++, addr = next, addr != end);
> +
> + /*
> + * We only change the mapping of the vmemmap virtual address range
> + * [@start + PAGE_SIZE, end), so we only need to flush the TLB which
> + * belongs to the range.
> + */
> + flush_tlb_kernel_range(start + PAGE_SIZE, end);
> +}
> +
> +/*
> + * Free a vmemmap page. A vmemmap page can be allocated from the memblock
> + * allocator or buddy allocator. If the PG_reserved flag is set, it means
> + * that it allocated from the memblock allocator, just free it via the
> + * free_bootmem_page(). Otherwise, use __free_page().
> + */
> +static inline void free_vmemmap_page(struct page *page)
> +{
> + if (PageReserved(page))
> + free_bootmem_page(page);
> + else
> + __free_page(page);
> +}
> +
> +/* Free a list of the vmemmap pages */
> +static void free_vmemmap_page_list(struct list_head *list)
> +{
> + struct page *page, *next;
> +
> + list_for_each_entry_safe(page, next, list, lru) {
> + list_del(&page->lru);
> + free_vmemmap_page(page);
> + }
> +}
> +
> +static void vmemmap_remap_pte(pte_t *pte, unsigned long addr,
> + struct vmemmap_remap_walk *walk)
> +{
> + /*
> + * Remap the tail pages as read-only to catch illegal write operation
> + * to the tail pages.
> + */
> + pgprot_t pgprot = PAGE_KERNEL_RO;
> + pte_t entry = mk_pte(walk->reuse_page, pgprot);
> + struct page *page = pte_page(*pte);
> +
> + list_add(&page->lru, walk->vmemmap_pages);
> + set_pte_at(&init_mm, addr, pte, entry);
> +}
> +
> +/**
> + * vmemmap_remap_free - remap the vmemmap virtual address range [@start, @end)
> + * to the page which @reuse is mapped to, then free vmemmap
> + * which the range are mapped to.
> + * @start: start address of the vmemmap virtual address range that we want
> + * to remap.
> + * @end: end address of the vmemmap virtual address range that we want to
> + * remap.
> + * @reuse: reuse address.
> + *
> + * Note: This function depends on vmemmap being base page mapped. Please make
> + * sure that we disable PMD mapping of vmemmap pages when calling this function.

This is something that the walking code enforces via BUG_ON's right?

> + */
> +void vmemmap_remap_free(unsigned long start, unsigned long end,
> + unsigned long reuse)
> +{
> + LIST_HEAD(vmemmap_pages);
> + struct vmemmap_remap_walk walk = {
> + .remap_pte = vmemmap_remap_pte,
> + .reuse_addr = reuse,
> + .vmemmap_pages = &vmemmap_pages,
> + };
> +
> + /*
> + * In order to make remapping routine most efficient for the huge pages,
> + * the routine of vmemmap page table walking has the following rules
> + * (see more details from the vmemmap_pte_range()):
> + *
> + * - The range [@start, @end) and the range [@reuse, @reuse + PAGE_SIZE)
> + * should be continuous.
> + * - The @reuse address is part of the range [@reuse, @end) that we are
> + * walking which is passed to vmemmap_remap_range().
> + * - The @reuse address is the first in the complete range.
> + *
> + * So we need to make sure that @start and @reuse meet the above rules.
> + */
> + BUG_ON(start - reuse != PAGE_SIZE);

Why even take a reuse arg then, just set reuse = start - PAGE_SIZE? If we do that
we can rename the function to reflect that the second page is reused or keep this
function and create an inline wrapper with reuse set to start - PAGE_SIZE and use
that for this use case and remove this BUG_ON

> +
> + vmemmap_remap_range(reuse, end, &walk);
> + free_vmemmap_page_list(&vmemmap_pages);
> +}
>
> /*
> * Allocate a block of memory to be used to back the virtual memory map
>



Balbir

2021-03-05 04:43:57

by Muchun Song

Subject: Re: [External] Re: [PATCH v17 3/9] mm: hugetlb: free the vmemmap pages associated with each HugeTLB page

On Fri, Mar 5, 2021 at 7:50 AM Singh, Balbir <[email protected]> wrote:
>
> On 26/2/21 12:21 am, Muchun Song wrote:
> > Every HugeTLB has more than one struct page structure. We __know__ that
> > we only use the first 4(HUGETLB_CGROUP_MIN_ORDER) struct page structures
> > to store metadata associated with each HugeTLB.
> >
> > There are a lot of struct page structures associated with each HugeTLB
> > page. For tail pages, the value of compound_head is the same. So we can
> > reuse first page of tail page structures. We map the virtual addresses
> > of the remaining pages of tail page structures to the first tail page
> > struct, and then free these page frames. Therefore, we need to reserve
> > two pages as vmemmap areas.
> >
> > When we allocate a HugeTLB page from the buddy, we can free some vmemmap
> > pages associated with each HugeTLB page. It is more appropriate to do it
> > in the prep_new_huge_page().
> >
> > The free_vmemmap_pages_per_hpage(), which indicates how many vmemmap
> > pages associated with a HugeTLB page can be freed, returns zero for
> > now, which means the feature is disabled. We will enable it once all
> > the infrastructure is there.
> >
> > Signed-off-by: Muchun Song <[email protected]>
> > Reviewed-by: Oscar Salvador <[email protected]>
> > ---
> > include/linux/bootmem_info.h | 27 +++++-
> > include/linux/mm.h | 3 +
> > mm/Makefile | 1 +
> > mm/hugetlb.c | 3 +
> > mm/hugetlb_vmemmap.c | 219 +++++++++++++++++++++++++++++++++++++++++++
> > mm/hugetlb_vmemmap.h | 20 ++++
> > mm/sparse-vmemmap.c | 207 ++++++++++++++++++++++++++++++++++++++++
> > 7 files changed, 479 insertions(+), 1 deletion(-)
> > create mode 100644 mm/hugetlb_vmemmap.c
> > create mode 100644 mm/hugetlb_vmemmap.h
> >
> > diff --git a/include/linux/bootmem_info.h b/include/linux/bootmem_info.h
> > index 4ed6dee1adc9..ec03a624dfa2 100644
> > --- a/include/linux/bootmem_info.h
> > +++ b/include/linux/bootmem_info.h
> > @@ -2,7 +2,7 @@
> > #ifndef __LINUX_BOOTMEM_INFO_H
> > #define __LINUX_BOOTMEM_INFO_H
> >
> > -#include <linux/mmzone.h>
> > +#include <linux/mm.h>
> >
> > /*
> > * Types for free bootmem stored in page->lru.next. These have to be in
> > @@ -22,6 +22,27 @@ void __init register_page_bootmem_info_node(struct pglist_data *pgdat);
> > void get_page_bootmem(unsigned long info, struct page *page,
> > unsigned long type);
> > void put_page_bootmem(struct page *page);
> > +
> > +/*
> > + * Any memory allocated via the memblock allocator and not via the
> > + * buddy will be marked reserved already in the memmap. For those
> > + * pages, we can call this function to free it to buddy allocator.
> > + */
> > +static inline void free_bootmem_page(struct page *page)
> > +{
> > + unsigned long magic = (unsigned long)page->freelist;
> > +
> > + /*
> > + * The reserve_bootmem_region sets the reserved flag on bootmem
> > + * pages.
> > + */
> > + VM_BUG_ON_PAGE(page_ref_count(page) != 2, page);
> > +
> > + if (magic == SECTION_INFO || magic == MIX_SECTION_INFO)
> > + put_page_bootmem(page);
> > + else
> > + VM_BUG_ON_PAGE(1, page);
> > +}
> > #else
> > static inline void register_page_bootmem_info_node(struct pglist_data *pgdat)
> > {
> > @@ -35,6 +56,10 @@ static inline void get_page_bootmem(unsigned long info, struct page *page,
> > unsigned long type)
> > {
> > }
> > +
> > +static inline void free_bootmem_page(struct page *page)
> > +{
> > +}
> > #endif
> >
> > #endif /* __LINUX_BOOTMEM_INFO_H */
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 77e64e3eac80..4ddfc31f21c6 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -2971,6 +2971,9 @@ static inline void print_vma_addr(char *prefix, unsigned long rip)
> > }
> > #endif
> >
> > +void vmemmap_remap_free(unsigned long start, unsigned long end,
> > + unsigned long reuse);
> > +
> > void *sparse_buffer_alloc(unsigned long size);
> > struct page * __populate_section_memmap(unsigned long pfn,
> > unsigned long nr_pages, int nid, struct vmem_altmap *altmap);
> > diff --git a/mm/Makefile b/mm/Makefile
> > index daabf86d7da8..3d7d57e3b55b 100644
> > --- a/mm/Makefile
> > +++ b/mm/Makefile
> > @@ -71,6 +71,7 @@ obj-$(CONFIG_FRONTSWAP) += frontswap.o
> > obj-$(CONFIG_ZSWAP) += zswap.o
> > obj-$(CONFIG_HAS_DMA) += dmapool.o
> > obj-$(CONFIG_HUGETLBFS) += hugetlb.o
> > +obj-$(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP) += hugetlb_vmemmap.o
> > obj-$(CONFIG_NUMA) += mempolicy.o
> > obj-$(CONFIG_SPARSEMEM) += sparse.o
> > obj-$(CONFIG_SPARSEMEM_VMEMMAP) += sparse-vmemmap.o
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index c232cb67dda2..43fed6785322 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -42,6 +42,7 @@
> > #include <linux/userfaultfd_k.h>
> > #include <linux/page_owner.h>
> > #include "internal.h"
> > +#include "hugetlb_vmemmap.h"
> >
> > int hugetlb_max_hstate __read_mostly;
> > unsigned int default_hstate_idx;
> > @@ -1463,6 +1464,8 @@ void free_huge_page(struct page *page)
> >
> > static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
> > {
> > + free_huge_page_vmemmap(h, page);
> > +
> > INIT_LIST_HEAD(&page->lru);
> > set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
> > set_hugetlb_cgroup(page, NULL);
> > diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> > new file mode 100644
> > index 000000000000..0209b736e0b4
> > --- /dev/null
> > +++ b/mm/hugetlb_vmemmap.c
> > @@ -0,0 +1,219 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/*
> > + * Free some vmemmap pages of HugeTLB
> > + *
> > + * Copyright (c) 2020, Bytedance. All rights reserved.
> > + *
> > + * Author: Muchun Song <[email protected]>
> > + *
> > + * The struct page structures (page structs) are used to describe a physical
> > + * page frame. By default, there is a one-to-one mapping from a page frame to
> > + * it's corresponding page struct.
> > + *
> > + * HugeTLB pages consist of multiple base page size pages and is supported by
> > + * many architectures. See hugetlbpage.rst in the Documentation directory for
> > + * more details. On the x86-64 architecture, HugeTLB pages of size 2MB and 1GB
> > + * are currently supported. Since the base page size on x86 is 4KB, a 2MB
> > + * HugeTLB page consists of 512 base pages and a 1GB HugeTLB page consists of
> > + * 4096 base pages. For each base page, there is a corresponding page struct.
> > + *
> > + * Within the HugeTLB subsystem, only the first 4 page structs are used to
> > + * contain unique information about a HugeTLB page. HUGETLB_CGROUP_MIN_ORDER
> > + * provides this upper limit. The only 'useful' information in the remaining
> > + * page structs is the compound_head field, and this field is the same for all
> > + * tail pages.
> > + *
> > + * By removing redundant page structs for HugeTLB pages, memory can be returned
> > + * to the buddy allocator for other uses.
> > + *
> > + * Different architectures support different HugeTLB pages. For example, the
> > + * following table is the HugeTLB page size supported by x86 and arm64
> > + * architectures. Because arm64 supports 4k, 16k, and 64k base pages and
> > + * supports contiguous entries, so it supports many kinds of sizes of HugeTLB
> > + * page.
> > + *
> > + * +--------------+-----------+-----------------------------------------------+
> > + * | Architecture | Page Size | HugeTLB Page Size |
> > + * +--------------+-----------+-----------+-----------+-----------+-----------+
> > + * | x86-64 | 4KB | 2MB | 1GB | | |
> > + * +--------------+-----------+-----------+-----------+-----------+-----------+
> > + * | | 4KB | 64KB | 2MB | 32MB | 1GB |
> > + * | +-----------+-----------+-----------+-----------+-----------+
> > + * | arm64 | 16KB | 2MB | 32MB | 1GB | |
> > + * | +-----------+-----------+-----------+-----------+-----------+
> > + * | | 64KB | 2MB | 512MB | 16GB | |
> > + * +--------------+-----------+-----------+-----------+-----------+-----------+
> > + *
> > + * When the system boot up, every HugeTLB page has more than one struct page
> > + * structs which size is (unit: pages):
> > + *
> > + * struct_size = HugeTLB_Size / PAGE_SIZE * sizeof(struct page) / PAGE_SIZE
> > + *
> > + * Where HugeTLB_Size is the size of the HugeTLB page. We know that the size
> > + * of the HugeTLB page is always n times PAGE_SIZE. So we can get the following
> > + * relationship.
> > + *
> > + * HugeTLB_Size = n * PAGE_SIZE
> > + *
> > + * Then,
> > + *
> > + * struct_size = n * PAGE_SIZE / PAGE_SIZE * sizeof(struct page) / PAGE_SIZE
> > + * = n * sizeof(struct page) / PAGE_SIZE
> > + *
> > + * We can use huge mapping at the pud/pmd level for the HugeTLB page.
> > + *
> > + * For the HugeTLB page of the pmd level mapping, then
> > + *
> > + * struct_size = n * sizeof(struct page) / PAGE_SIZE
> > + * = PAGE_SIZE / sizeof(pte_t) * sizeof(struct page) / PAGE_SIZE
> > + * = sizeof(struct page) / sizeof(pte_t)
> > + * = 64 / 8
> > + * = 8 (pages)
> > + *
> > + * Where n is how many pte entries which one page can contains. So the value of
> > + * n is (PAGE_SIZE / sizeof(pte_t)).
> > + *
> > + * This optimization only supports 64-bit system, so the value of sizeof(pte_t)
> > + * is 8. And this optimization also applicable only when the size of struct page
> > + * is a power of two. In most cases, the size of struct page is 64 bytes (e.g.
> > + * x86-64 and arm64). So if we use pmd level mapping for a HugeTLB page, the
> > + * size of struct page structs of it is 8 page frames which size depends on the
> > + * size of the base page.
> > + *
> > + * For the HugeTLB page of the pud level mapping, then
> > + *
> > + * struct_size = PAGE_SIZE / sizeof(pmd_t) * struct_size(pmd)
> > + * = PAGE_SIZE / 8 * 8 (pages)
> > + * = PAGE_SIZE (pages)
> > + *
> > + * Where the struct_size(pmd) is the size of the struct page structs of a
> > + * HugeTLB page of the pmd level mapping.
> > + *
> > + * E.g.: A 2MB HugeTLB page on x86_64 consists in 8 page frames while 1GB
> > + * HugeTLB page consists in 4096.
> > + *
> > + * Next, we take the pmd level mapping of the HugeTLB page as an example to
> > + * show the internal implementation of this optimization. There are 8 pages
> > + * struct page structs associated with a HugeTLB page which is pmd mapped.
> > + *
> > + * Here is how things look before optimization.
> > + *
> > + * HugeTLB struct pages(8 pages) page frame(8 pages)
> > + * +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+
> > + * | | | 0 | -------------> | 0 |
> > + * | | +-----------+ +-----------+
> > + * | | | 1 | -------------> | 1 |
> > + * | | +-----------+ +-----------+
> > + * | | | 2 | -------------> | 2 |
> > + * | | +-----------+ +-----------+
> > + * | | | 3 | -------------> | 3 |
> > + * | | +-----------+ +-----------+
> > + * | | | 4 | -------------> | 4 |
> > + * | PMD | +-----------+ +-----------+
> > + * | level | | 5 | -------------> | 5 |
> > + * | mapping | +-----------+ +-----------+
> > + * | | | 6 | -------------> | 6 |
> > + * | | +-----------+ +-----------+
> > + * | | | 7 | -------------> | 7 |
> > + * | | +-----------+ +-----------+
> > + * | |
> > + * | |
> > + * | |
> > + * +-----------+
> > + *
> > + * The value of page->compound_head is the same for all tail pages. The first
> > + * page of page structs (page 0) associated with the HugeTLB page contains the 4
> > + * page structs necessary to describe the HugeTLB. The only use of the remaining
> > + * pages of page structs (page 1 to page 7) is to point to page->compound_head.
> > + * Therefore, we can remap pages 2 to 7 to page 1. Only 2 pages of page structs
> > + * will be used for each HugeTLB page. This will allow us to free the remaining
> > + * 6 pages to the buddy allocator.
> > + *
> > + * Here is how things look after remapping.
> > + *
> > + * HugeTLB struct pages(8 pages) page frame(8 pages)
> > + * +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+
> > + * | | | 0 | -------------> | 0 |
> > + * | | +-----------+ +-----------+
> > + * | | | 1 | -------------> | 1 |
> > + * | | +-----------+ +-----------+
> > + * | | | 2 | ----------------^ ^ ^ ^ ^ ^
> > + * | | +-----------+ | | | | |
> > + * | | | 3 | ------------------+ | | | |
> > + * | | +-----------+ | | | |
> > + * | | | 4 | --------------------+ | | |
> > + * | PMD | +-----------+ | | |
> > + * | level | | 5 | ----------------------+ | |
> > + * | mapping | +-----------+ | |
> > + * | | | 6 | ------------------------+ |
> > + * | | +-----------+ |
> > + * | | | 7 | --------------------------+
> > + * | | +-----------+
> > + * | |
> > + * | |
> > + * | |
> > + * +-----------+
> > + *
> > + * When a HugeTLB is freed to the buddy system, we should allocate 6 pages for
> > + * vmemmap pages and restore the previous mapping relationship.
> > + *
> > + * For the HugeTLB page of the pud level mapping. It is similar to the former.
> > + * We also can use this approach to free (PAGE_SIZE - 2) vmemmap pages.
> > + *
> > + * Apart from the HugeTLB page of the pmd/pud level mapping, some architectures
> > + * (e.g. aarch64) provides a contiguous bit in the translation table entries
> > + * that hints to the MMU to indicate that it is one of a contiguous set of
> > + * entries that can be cached in a single TLB entry.
> > + *
> > + * The contiguous bit is used to increase the mapping size at the pmd and pte
> > + * (last) level. So this type of HugeTLB page can be optimized only when its
> > + * size of the struct page structs is greater than 2 pages.
> > + */
> > +#include "hugetlb_vmemmap.h"
> > +
> > +/*
> > + * There are a lot of struct page structures associated with each HugeTLB page.
> > + * For tail pages, the value of compound_head is the same. So we can reuse first
> > + * page of tail page structures. We map the virtual addresses of the remaining
> > + * pages of tail page structures to the first tail page struct, and then free
> > + * these page frames. Therefore, we need to reserve two pages as vmemmap areas.
> > + */
> > +#define RESERVE_VMEMMAP_NR 2U
> > +#define RESERVE_VMEMMAP_SIZE (RESERVE_VMEMMAP_NR << PAGE_SHIFT)
> > +
> > +/*
> > + * How many vmemmap pages associated with a HugeTLB page that can be freed
> > + * to the buddy allocator.
> > + *
> > + * Todo: Returns zero for now, which means the feature is disabled. We will
> > + * enable it once all the infrastructure is there.
> > + */
> > +static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
> > +{
> > + return 0;
> > +}
> > +
> > +static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h)
> > +{
> > + return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT;
> > +}
> > +
> > +void free_huge_page_vmemmap(struct hstate *h, struct page *head)
> > +{
> > + unsigned long vmemmap_addr = (unsigned long)head;
> > + unsigned long vmemmap_end, vmemmap_reuse;
> > +
> > + if (!free_vmemmap_pages_per_hpage(h))
> > + return;
> > +
> > + vmemmap_addr += RESERVE_VMEMMAP_SIZE;
> > + vmemmap_end = vmemmap_addr + free_vmemmap_pages_size_per_hpage(h);
> > + vmemmap_reuse = vmemmap_addr - PAGE_SIZE;
> > +
> > + /*
> > + * Remap the vmemmap virtual address range [@vmemmap_addr, @vmemmap_end)
> > + * to the page which @vmemmap_reuse is mapped to, then free the pages
> > + * which the range [@vmemmap_addr, @vmemmap_end] is mapped to.
> > + */
> > + vmemmap_remap_free(vmemmap_addr, vmemmap_end, vmemmap_reuse);
> > +}
> > diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
> > new file mode 100644
> > index 000000000000..6923f03534d5
> > --- /dev/null
> > +++ b/mm/hugetlb_vmemmap.h
> > @@ -0,0 +1,20 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/*
> > + * Free some vmemmap pages of HugeTLB
> > + *
> > + * Copyright (c) 2020, Bytedance. All rights reserved.
> > + *
> > + * Author: Muchun Song <[email protected]>
> > + */
> > +#ifndef _LINUX_HUGETLB_VMEMMAP_H
> > +#define _LINUX_HUGETLB_VMEMMAP_H
> > +#include <linux/hugetlb.h>
> > +
> > +#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> > +void free_huge_page_vmemmap(struct hstate *h, struct page *head);
> > +#else
> > +static inline void free_huge_page_vmemmap(struct hstate *h, struct page *head)
> > +{
> > +}
> > +#endif /* CONFIG_HUGETLB_PAGE_FREE_VMEMMAP */
> > +#endif /* _LINUX_HUGETLB_VMEMMAP_H */
> > diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> > index 16183d85a7d5..d3076a7a3783 100644
> > --- a/mm/sparse-vmemmap.c
> > +++ b/mm/sparse-vmemmap.c
> > @@ -27,8 +27,215 @@
> > #include <linux/spinlock.h>
> > #include <linux/vmalloc.h>
> > #include <linux/sched.h>
> > +#include <linux/pgtable.h>
> > +#include <linux/bootmem_info.h>
> > +
> > #include <asm/dma.h>
> > #include <asm/pgalloc.h>
> > +#include <asm/tlbflush.h>
> > +
> > +/**
> > + * vmemmap_remap_walk - walk vmemmap page table
> > + *
> > + * @remap_pte: called for each lowest-level entry (PTE).
> > + * @reuse_page: the page which is reused for the tail vmemmap pages.
> > + * @reuse_addr: the virtual address of the @reuse_page page.
> > + * @vmemmap_pages: the list head of the vmemmap pages that can be freed.
> > + */
> > +struct vmemmap_remap_walk {
> > + void (*remap_pte)(pte_t *pte, unsigned long addr,
> > + struct vmemmap_remap_walk *walk);
> > + struct page *reuse_page;
> > + unsigned long reuse_addr;
> > + struct list_head *vmemmap_pages;
> > +};
> > +
> > +static void vmemmap_pte_range(pmd_t *pmd, unsigned long addr,
> > + unsigned long end,
> > + struct vmemmap_remap_walk *walk)
> > +{
> > + pte_t *pte;
> > +
> > + pte = pte_offset_kernel(pmd, addr);
> > +
> > + /*
> > + * The reuse_page is found 'first' in table walk before we start
> > + * remapping (which is calling @walk->remap_pte).
> > + */
> > + if (!walk->reuse_page) {
> > + BUG_ON(pte_none(*pte));
> > + BUG_ON(walk->reuse_addr != addr);
> > +
> > + walk->reuse_page = pte_page(*pte++);
>
> The concurrency semantics of this code are not clear, do we need READ_ONCE()/
> WRITE_ONCE() semantics if this page walk is lockless? Can we run this code
> in parallel on the same section? I presume not

IIUC, there is no parallel thread to walk the page tables of the
vmemmap area. We may not need READ_ONCE/WRITE_ONCE.

>
> > + /*
> > + * Because the reuse address is part of the range that we are
> > + * walking, skip the reuse address range.
> > + */
> > + addr += PAGE_SIZE;
> > + }
> > +
> > + for (; addr != end; addr += PAGE_SIZE, pte++) {
> > + BUG_ON(pte_none(*pte));
> > +
> > + walk->remap_pte(pte, addr, walk);
> > + }
> > +}
> > +
> > +static void vmemmap_pmd_range(pud_t *pud, unsigned long addr,
> > + unsigned long end,
> > + struct vmemmap_remap_walk *walk)
> > +{
> > + pmd_t *pmd;
> > + unsigned long next;
> > +
> > + pmd = pmd_offset(pud, addr);
> > + do {
> > + BUG_ON(pmd_none(*pmd) || pmd_leaf(*pmd));
> > +
> > + next = pmd_addr_end(addr, end);
> > + vmemmap_pte_range(pmd, addr, next, walk);
> > + } while (pmd++, addr = next, addr != end);
> > +}
> > +
> > +static void vmemmap_pud_range(p4d_t *p4d, unsigned long addr,
> > + unsigned long end,
> > + struct vmemmap_remap_walk *walk)
> > +{
> > + pud_t *pud;
> > + unsigned long next;
> > +
> > + pud = pud_offset(p4d, addr);
> > + do {
> > + BUG_ON(pud_none(*pud));
> > +
> > + next = pud_addr_end(addr, end);
> > + vmemmap_pmd_range(pud, addr, next, walk);
> > + } while (pud++, addr = next, addr != end);
> > +}
> > +
> > +static void vmemmap_p4d_range(pgd_t *pgd, unsigned long addr,
> > + unsigned long end,
> > + struct vmemmap_remap_walk *walk)
> > +{
> > + p4d_t *p4d;
> > + unsigned long next;
> > +
> > + p4d = p4d_offset(pgd, addr);
> > + do {
> > + BUG_ON(p4d_none(*p4d));
> > +
> > + next = p4d_addr_end(addr, end);
> > + vmemmap_pud_range(p4d, addr, next, walk);
> > + } while (p4d++, addr = next, addr != end);
> > +}
> > +
> > +static void vmemmap_remap_range(unsigned long start, unsigned long end,
> > + struct vmemmap_remap_walk *walk)
> > +{
> > + unsigned long addr = start;
> > + unsigned long next;
> > + pgd_t *pgd;
> > +
> > + VM_BUG_ON(!IS_ALIGNED(start, PAGE_SIZE));
> > + VM_BUG_ON(!IS_ALIGNED(end, PAGE_SIZE));
> > +
> > + pgd = pgd_offset_k(addr);
> > + do {
> > + BUG_ON(pgd_none(*pgd));
> > +
> > + next = pgd_addr_end(addr, end);
> > + vmemmap_p4d_range(pgd, addr, next, walk);
> > + } while (pgd++, addr = next, addr != end);
> > +
> > + /*
> > + * We only change the mapping of the vmemmap virtual address range
> > + * [@start + PAGE_SIZE, end), so we only need to flush the TLB which
> > + * belongs to the range.
> > + */
> > + flush_tlb_kernel_range(start + PAGE_SIZE, end);
> > +}
> > +
> > +/*
> > + * Free a vmemmap page. A vmemmap page can be allocated from the memblock
> > + * allocator or buddy allocator. If the PG_reserved flag is set, it means
> > + * that it allocated from the memblock allocator, just free it via the
> > + * free_bootmem_page(). Otherwise, use __free_page().
> > + */
> > +static inline void free_vmemmap_page(struct page *page)
> > +{
> > + if (PageReserved(page))
> > + free_bootmem_page(page);
> > + else
> > + __free_page(page);
> > +}
> > +
> > +/* Free a list of the vmemmap pages */
> > +static void free_vmemmap_page_list(struct list_head *list)
> > +{
> > + struct page *page, *next;
> > +
> > + list_for_each_entry_safe(page, next, list, lru) {
> > + list_del(&page->lru);
> > + free_vmemmap_page(page);
> > + }
> > +}
> > +
> > +static void vmemmap_remap_pte(pte_t *pte, unsigned long addr,
> > + struct vmemmap_remap_walk *walk)
> > +{
> > + /*
> > + * Remap the tail pages as read-only to catch illegal write operation
> > + * to the tail pages.
> > + */
> > + pgprot_t pgprot = PAGE_KERNEL_RO;
> > + pte_t entry = mk_pte(walk->reuse_page, pgprot);
> > + struct page *page = pte_page(*pte);
> > +
> > + list_add(&page->lru, walk->vmemmap_pages);
> > + set_pte_at(&init_mm, addr, pte, entry);
> > +}
> > +
> > +/**
> > + * vmemmap_remap_free - remap the vmemmap virtual address range [@start, @end)
> > + * to the page which @reuse is mapped to, then free vmemmap
> > + * which the range are mapped to.
> > + * @start: start address of the vmemmap virtual address range that we want
> > + * to remap.
> > + * @end: end address of the vmemmap virtual address range that we want to
> > + * remap.
> > + * @reuse: reuse address.
> > + *
> > + * Note: This function depends on vmemmap being base page mapped. Please make
> > + * sure that we disable PMD mapping of vmemmap pages when calling this function.
>
> This is something that the walking code enforces via BUG_ON's right?

Right. There is a BUG_ON(pmd_leaf(*pmd)) in vmemmap_pmd_range().

>
> > + */
> > +void vmemmap_remap_free(unsigned long start, unsigned long end,
> > + unsigned long reuse)
> > +{
> > + LIST_HEAD(vmemmap_pages);
> > + struct vmemmap_remap_walk walk = {
> > + .remap_pte = vmemmap_remap_pte,
> > + .reuse_addr = reuse,
> > + .vmemmap_pages = &vmemmap_pages,
> > + };
> > +
> > + /*
> > + * In order to make remapping routine most efficient for the huge pages,
> > + * the routine of vmemmap page table walking has the following rules
> > + * (see more details from the vmemmap_pte_range()):
> > + *
> > + * - The range [@start, @end) and the range [@reuse, @reuse + PAGE_SIZE)
> > + * should be continuous.
> > + * - The @reuse address is part of the range [@reuse, @end) that we are
> > + * walking which is passed to vmemmap_remap_range().
> > + * - The @reuse address is the first in the complete range.
> > + *
> > + * So we need to make sure that @start and @reuse meet the above rules.
> > + */
> > + BUG_ON(start - reuse != PAGE_SIZE);
>
> Why even take a reuse arg then, just set reuse = start - PAGE_SIZE? If we do that
> we can rename the function to reflect that the second page is reused or

There was a discussion about "why we introduce reuse parameter" in a
previous version of the series starting here:

https://patchwork.kernel.org/project/linux-mm/patch/[email protected]/

> keep this
> function and create an inline wrapper with reuse set to start - PAGE_SIZE and use
> that for this use case and remove this BUG_ON

I also want to hear Oscar and Mike's suggestions about this.

Thanks.

>
> > +
> > + vmemmap_remap_range(reuse, end, &walk);
> > + free_vmemmap_page_list(&vmemmap_pages);
> > +}
> >
> > /*
> > * Allocate a block of memory to be used to back the virtual memory map
> >
>
>
>
> Balbir

2021-03-05 08:57:45

by Balbir Singh

Subject: Re: [PATCH v17 4/9] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page

On Thu, Feb 25, 2021 at 09:21:25PM +0800, Muchun Song wrote:
> When we free a HugeTLB page to the buddy allocator, we should allocate
> the vmemmap pages associated with it. But we may cannot allocate vmemmap
> pages when the system is under memory pressure, in this case, we just
> refuse to free the HugeTLB page instead of looping forever trying to
> allocate the pages. This changes some behavior (list below) on some
> corner cases.
>
> 1) Failing to free a huge page triggered by the user (decrease nr_pages).
>
> Need try again later by the user.
>
> 2) Failing to free a surplus huge page when freed by the application.
>
> Try again later when freeing a huge page next time.
>
> 3) Failing to dissolve a free huge page on ZONE_MOVABLE via
> offline_pages().
>
> This is a bit unfortunate if we have plenty of ZONE_MOVABLE memory
> but are low on kernel memory. For example, migration of huge pages
> would still work, however, dissolving the free page does not work.
> This is a corner cases. When the system is that much under memory
> pressure, offlining/unplug can be expected to fail. This is
> unfortunate because it prevents from the memory offlining which
> shouldn't happen for movable zones. People depending on the memory
> hotplug and movable zone should carefuly consider whether savings
> on unmovable memory are worth losing their hotplug functionality
> in some situations.
>
> 4) Failing to dissolve a huge page on CMA/ZONE_MOVABLE via
> alloc_contig_range() - once we have that handling in place. Mainly
> affects CMA and virtio-mem.
>
> Similar to 3). virito-mem will handle migration errors gracefully.
> CMA might be able to fallback on other free areas within the CMA
> region.
>
> Vmemmap pages are allocated from the page freeing context. In order for
> those allocations to be not disruptive (e.g. trigger oom killer)
> __GFP_NORETRY is used. hugetlb_lock is dropped for the allocation
> because a non sleeping allocation would be too fragile and it could fail
> too easily under memory pressure. GFP_ATOMIC or other modes to access
> memory reserves is not used because we want to prevent consuming
> reserves under heavy hugetlb freeing.
>
> Signed-off-by: Muchun Song <[email protected]>
> ---
> Documentation/admin-guide/mm/hugetlbpage.rst | 8 +++
> include/linux/mm.h | 2 +
> mm/hugetlb.c | 92 +++++++++++++++++++++-------
> mm/hugetlb_vmemmap.c | 32 ++++++----
> mm/hugetlb_vmemmap.h | 23 +++++++
> mm/sparse-vmemmap.c | 75 ++++++++++++++++++++++-
> 6 files changed, 197 insertions(+), 35 deletions(-)
>
> diff --git a/Documentation/admin-guide/mm/hugetlbpage.rst b/Documentation/admin-guide/mm/hugetlbpage.rst
> index f7b1c7462991..6988895d09a8 100644
> --- a/Documentation/admin-guide/mm/hugetlbpage.rst
> +++ b/Documentation/admin-guide/mm/hugetlbpage.rst
> @@ -60,6 +60,10 @@ HugePages_Surp
> the pool above the value in ``/proc/sys/vm/nr_hugepages``. The
> maximum number of surplus huge pages is controlled by
> ``/proc/sys/vm/nr_overcommit_hugepages``.
> + Note: When the feature of freeing unused vmemmap pages associated
> + with each hugetlb page is enabled, the number of surplus huge pages
> + may be temporarily larger than the maximum number of surplus huge
> + pages when the system is under memory pressure.
> Hugepagesize
> is the default hugepage size (in Kb).
> Hugetlb
> @@ -80,6 +84,10 @@ returned to the huge page pool when freed by a task. A user with root
> privileges can dynamically allocate more or free some persistent huge pages
> by increasing or decreasing the value of ``nr_hugepages``.
>
> +Note: When the feature of freeing unused vmemmap pages associated with each
> +hugetlb page is enabled, we can fail to free the huge pages triggered by
> +the user when ths system is under memory pressure. Please try again later.
> +
> Pages that are used as huge pages are reserved inside the kernel and cannot
> be used for other purposes. Huge pages cannot be swapped out under
> memory pressure.
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 4ddfc31f21c6..77693c944a36 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2973,6 +2973,8 @@ static inline void print_vma_addr(char *prefix, unsigned long rip)
>
> void vmemmap_remap_free(unsigned long start, unsigned long end,
> unsigned long reuse);
> +int vmemmap_remap_alloc(unsigned long start, unsigned long end,
> + unsigned long reuse, gfp_t gfp_mask);
>
> void *sparse_buffer_alloc(unsigned long size);
> struct page * __populate_section_memmap(unsigned long pfn,
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 43fed6785322..b6e4e3f31ad2 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1304,16 +1304,59 @@ static inline void destroy_compound_gigantic_page(struct page *page,
> unsigned int order) { }
> #endif
>
> -static void update_and_free_page(struct hstate *h, struct page *page)
> +static int update_and_free_page(struct hstate *h, struct page *page)
> + __releases(&hugetlb_lock) __acquires(&hugetlb_lock)
> {
> int i;
> struct page *subpage = page;
> + int nid = page_to_nid(page);
>
> if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
> - return;
> + return 0;
>
> h->nr_huge_pages--;
> - h->nr_huge_pages_node[page_to_nid(page)]--;
> + h->nr_huge_pages_node[nid]--;
> + VM_BUG_ON_PAGE(hugetlb_cgroup_from_page(page), page);
> + VM_BUG_ON_PAGE(hugetlb_cgroup_from_page_rsvd(page), page);
> + set_page_refcounted(page);
> + set_compound_page_dtor(page, NULL_COMPOUND_DTOR);
> +
> + /*
> + * If the vmemmap pages associated with the HugeTLB page can be
> + * optimized or the page is gigantic, we might block in
> + * alloc_huge_page_vmemmap() or free_gigantic_page(). In both
> + * cases, drop the hugetlb_lock.
> + */
> + if (free_vmemmap_pages_per_hpage(h) || hstate_is_gigantic(h))
> + spin_unlock(&hugetlb_lock);
> +
> + if (alloc_huge_page_vmemmap(h, page)) {
> + spin_lock(&hugetlb_lock);
> + INIT_LIST_HEAD(&page->lru);
> + set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
> + h->nr_huge_pages++;
> + h->nr_huge_pages_node[nid]++;
> +
> + /*
> + * If we cannot allocate vmemmap pages, just refuse to free the
> + * page and put the page back on the hugetlb free list and treat
> + * as a surplus page.
> + */
> + h->surplus_huge_pages++;
> + h->surplus_huge_pages_node[nid]++;
> +
> + /*
> + * The refcount can be perfectly increased by memory-failure or
> + * soft_offline handlers.
> + */
> + if (likely(put_page_testzero(page))) {
> + arch_clear_hugepage_flags(page);
> + enqueue_huge_page(h, page);
> + }
> +
> + return -ENOMEM;
> + }
> +
> for (i = 0; i < pages_per_huge_page(h);
> i++, subpage = mem_map_next(subpage, page, i)) {
> subpage->flags &= ~(1 << PG_locked | 1 << PG_error |
> @@ -1321,22 +1364,18 @@ static void update_and_free_page(struct hstate *h, struct page *page)
> 1 << PG_active | 1 << PG_private |
> 1 << PG_writeback);
> }
> - VM_BUG_ON_PAGE(hugetlb_cgroup_from_page(page), page);
> - VM_BUG_ON_PAGE(hugetlb_cgroup_from_page_rsvd(page), page);
> - set_compound_page_dtor(page, NULL_COMPOUND_DTOR);
> - set_page_refcounted(page);
> +
> if (hstate_is_gigantic(h)) {
> - /*
> - * Temporarily drop the hugetlb_lock, because
> - * we might block in free_gigantic_page().
> - */
> - spin_unlock(&hugetlb_lock);
> destroy_compound_gigantic_page(page, huge_page_order(h));
> free_gigantic_page(page, huge_page_order(h));
> - spin_lock(&hugetlb_lock);
> } else {
> __free_pages(page, huge_page_order(h));
> }
> +
> + if (free_vmemmap_pages_per_hpage(h) || hstate_is_gigantic(h))
> + spin_lock(&hugetlb_lock);
> +
> + return 0;
> }
>
> struct hstate *size_to_hstate(unsigned long size)
> @@ -1404,9 +1443,9 @@ static void __free_huge_page(struct page *page)
> } else if (h->surplus_huge_pages_node[nid]) {
> /* remove the page from active list */
> list_del(&page->lru);
> - update_and_free_page(h, page);
> h->surplus_huge_pages--;
> h->surplus_huge_pages_node[nid]--;
> + update_and_free_page(h, page);
> } else {
> arch_clear_hugepage_flags(page);
> enqueue_huge_page(h, page);
> @@ -1447,7 +1486,7 @@ void free_huge_page(struct page *page)
> /*
> * Defer freeing if in non-task context to avoid hugetlb_lock deadlock.
> */
> - if (!in_task()) {
> + if (!in_atomic()) {
> /*
> * Only call schedule_work() if hpage_freelist is previously
> * empty. Otherwise, schedule_work() had been called but the
> @@ -1699,8 +1738,7 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed,
> h->surplus_huge_pages--;
> h->surplus_huge_pages_node[node]--;
> }
> - update_and_free_page(h, page);
> - ret = 1;
> + ret = !update_and_free_page(h, page);
> break;
> }
> }
> @@ -1713,10 +1751,14 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed,
> * nothing for in-use hugepages and non-hugepages.
> * This function returns values like below:
> *
> - * -EBUSY: failed to dissolved free hugepages or the hugepage is in-use
> - * (allocated or reserved.)
> - * 0: successfully dissolved free hugepages or the page is not a
> - * hugepage (considered as already dissolved)
> + * -ENOMEM: failed to allocate vmemmap pages to free the freed hugepages
> + * when the system is under memory pressure and the feature of
> + * freeing unused vmemmap pages associated with each hugetlb page
> + * is enabled.
> + * -EBUSY: failed to dissolved free hugepages or the hugepage is in-use
> + * (allocated or reserved.)
> + * 0: successfully dissolved free hugepages or the page is not a
> + * hugepage (considered as already dissolved)
> */
> int dissolve_free_huge_page(struct page *page)
> {
> @@ -1771,8 +1813,12 @@ int dissolve_free_huge_page(struct page *page)
> h->free_huge_pages--;
> h->free_huge_pages_node[nid]--;
> h->max_huge_pages--;
> - update_and_free_page(h, head);
> - rc = 0;
> + rc = update_and_free_page(h, head);
> + if (rc) {
> + h->surplus_huge_pages--;
> + h->surplus_huge_pages_node[nid]--;
> + h->max_huge_pages++;
> + }
> }
> out:
> spin_unlock(&hugetlb_lock);
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index 0209b736e0b4..f7ab3d99250a 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -181,21 +181,31 @@
> #define RESERVE_VMEMMAP_NR 2U
> #define RESERVE_VMEMMAP_SIZE (RESERVE_VMEMMAP_NR << PAGE_SHIFT)
>
> -/*
> - * How many vmemmap pages associated with a HugeTLB page that can be freed
> - * to the buddy allocator.
> - *
> - * Todo: Returns zero for now, which means the feature is disabled. We will
> - * enable it once all the infrastructure is there.
> - */
> -static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
> +static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h)
> {
> - return 0;
> + return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT;
> }
>
> -static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h)
> +int alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
> {
> - return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT;
> + unsigned long vmemmap_addr = (unsigned long)head;
> + unsigned long vmemmap_end, vmemmap_reuse;
> +
> + if (!free_vmemmap_pages_per_hpage(h))
> + return 0;
> +
> + vmemmap_addr += RESERVE_VMEMMAP_SIZE;
> + vmemmap_end = vmemmap_addr + free_vmemmap_pages_size_per_hpage(h);
> + vmemmap_reuse = vmemmap_addr - PAGE_SIZE;

This is where I think some optimization is possible, once we are done with
vmemmap_end calculation, we can use 6 pages (for 2MiB huge page) as pages
for struct page. Is there a reason to not do so?

Balbir

2021-03-05 09:33:29

by Muchun Song

Subject: Re: [External] Re: [PATCH v17 4/9] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page

On Fri, Mar 5, 2021 at 4:55 PM Balbir Singh <[email protected]> wrote:
>
> On Thu, Feb 25, 2021 at 09:21:25PM +0800, Muchun Song wrote:
> > When we free a HugeTLB page to the buddy allocator, we should allocate
> > the vmemmap pages associated with it. But we may cannot allocate vmemmap
> > pages when the system is under memory pressure, in this case, we just
> > refuse to free the HugeTLB page instead of looping forever trying to
> > allocate the pages. This changes some behavior (list below) on some
> > corner cases.
> >
> > 1) Failing to free a huge page triggered by the user (decrease nr_pages).
> >
> > Need try again later by the user.
> >
> > 2) Failing to free a surplus huge page when freed by the application.
> >
> > Try again later when freeing a huge page next time.
> >
> > 3) Failing to dissolve a free huge page on ZONE_MOVABLE via
> > offline_pages().
> >
> > This is a bit unfortunate if we have plenty of ZONE_MOVABLE memory
> > but are low on kernel memory. For example, migration of huge pages
> > would still work, however, dissolving the free page does not work.
> > This is a corner cases. When the system is that much under memory
> > pressure, offlining/unplug can be expected to fail. This is
> > unfortunate because it prevents from the memory offlining which
> > shouldn't happen for movable zones. People depending on the memory
> > hotplug and movable zone should carefuly consider whether savings
> > on unmovable memory are worth losing their hotplug functionality
> > in some situations.
> >
> > 4) Failing to dissolve a huge page on CMA/ZONE_MOVABLE via
> > alloc_contig_range() - once we have that handling in place. Mainly
> > affects CMA and virtio-mem.
> >
> > Similar to 3). virito-mem will handle migration errors gracefully.
> > CMA might be able to fallback on other free areas within the CMA
> > region.
> >
> > Vmemmap pages are allocated from the page freeing context. In order for
> > those allocations to be not disruptive (e.g. trigger oom killer)
> > __GFP_NORETRY is used. hugetlb_lock is dropped for the allocation
> > because a non sleeping allocation would be too fragile and it could fail
> > too easily under memory pressure. GFP_ATOMIC or other modes to access
> > memory reserves is not used because we want to prevent consuming
> > reserves under heavy hugetlb freeing.
> >
> > Signed-off-by: Muchun Song <[email protected]>
> > ---
> > Documentation/admin-guide/mm/hugetlbpage.rst | 8 +++
> > include/linux/mm.h | 2 +
> > mm/hugetlb.c | 92 +++++++++++++++++++++-------
> > mm/hugetlb_vmemmap.c | 32 ++++++----
> > mm/hugetlb_vmemmap.h | 23 +++++++
> > mm/sparse-vmemmap.c | 75 ++++++++++++++++++++++-
> > 6 files changed, 197 insertions(+), 35 deletions(-)
> >
> > diff --git a/Documentation/admin-guide/mm/hugetlbpage.rst b/Documentation/admin-guide/mm/hugetlbpage.rst
> > index f7b1c7462991..6988895d09a8 100644
> > --- a/Documentation/admin-guide/mm/hugetlbpage.rst
> > +++ b/Documentation/admin-guide/mm/hugetlbpage.rst
> > @@ -60,6 +60,10 @@ HugePages_Surp
> > the pool above the value in ``/proc/sys/vm/nr_hugepages``. The
> > maximum number of surplus huge pages is controlled by
> > ``/proc/sys/vm/nr_overcommit_hugepages``.
> > + Note: When the feature of freeing unused vmemmap pages associated
> > + with each hugetlb page is enabled, the number of surplus huge pages
> > + may be temporarily larger than the maximum number of surplus huge
> > + pages when the system is under memory pressure.
> > Hugepagesize
> > is the default hugepage size (in Kb).
> > Hugetlb
> > @@ -80,6 +84,10 @@ returned to the huge page pool when freed by a task. A user with root
> > privileges can dynamically allocate more or free some persistent huge pages
> > by increasing or decreasing the value of ``nr_hugepages``.
> >
> > +Note: When the feature of freeing unused vmemmap pages associated with each
> > +hugetlb page is enabled, we can fail to free the huge pages triggered by
> > +the user when ths system is under memory pressure. Please try again later.
> > +
> > Pages that are used as huge pages are reserved inside the kernel and cannot
> > be used for other purposes. Huge pages cannot be swapped out under
> > memory pressure.
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 4ddfc31f21c6..77693c944a36 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -2973,6 +2973,8 @@ static inline void print_vma_addr(char *prefix, unsigned long rip)
> >
> > void vmemmap_remap_free(unsigned long start, unsigned long end,
> > unsigned long reuse);
> > +int vmemmap_remap_alloc(unsigned long start, unsigned long end,
> > + unsigned long reuse, gfp_t gfp_mask);
> >
> > void *sparse_buffer_alloc(unsigned long size);
> > struct page * __populate_section_memmap(unsigned long pfn,
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 43fed6785322..b6e4e3f31ad2 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -1304,16 +1304,59 @@ static inline void destroy_compound_gigantic_page(struct page *page,
> > unsigned int order) { }
> > #endif
> >
> > -static void update_and_free_page(struct hstate *h, struct page *page)
> > +static int update_and_free_page(struct hstate *h, struct page *page)
> > + __releases(&hugetlb_lock) __acquires(&hugetlb_lock)
> > {
> > int i;
> > struct page *subpage = page;
> > + int nid = page_to_nid(page);
> >
> > if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
> > - return;
> > + return 0;
> >
> > h->nr_huge_pages--;
> > - h->nr_huge_pages_node[page_to_nid(page)]--;
> > + h->nr_huge_pages_node[nid]--;
> > + VM_BUG_ON_PAGE(hugetlb_cgroup_from_page(page), page);
> > + VM_BUG_ON_PAGE(hugetlb_cgroup_from_page_rsvd(page), page);
> > + set_page_refcounted(page);
> > + set_compound_page_dtor(page, NULL_COMPOUND_DTOR);
> > +
> > + /*
> > + * If the vmemmap pages associated with the HugeTLB page can be
> > + * optimized or the page is gigantic, we might block in
> > + * alloc_huge_page_vmemmap() or free_gigantic_page(). In both
> > + * cases, drop the hugetlb_lock.
> > + */
> > + if (free_vmemmap_pages_per_hpage(h) || hstate_is_gigantic(h))
> > + spin_unlock(&hugetlb_lock);
> > +
> > + if (alloc_huge_page_vmemmap(h, page)) {
> > + spin_lock(&hugetlb_lock);
> > + INIT_LIST_HEAD(&page->lru);
> > + set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
> > + h->nr_huge_pages++;
> > + h->nr_huge_pages_node[nid]++;
> > +
> > + /*
> > + * If we cannot allocate vmemmap pages, just refuse to free the
> > + * page and put the page back on the hugetlb free list and treat
> > + * as a surplus page.
> > + */
> > + h->surplus_huge_pages++;
> > + h->surplus_huge_pages_node[nid]++;
> > +
> > + /*
> > + * The refcount can be perfectly increased by memory-failure or
> > + * soft_offline handlers.
> > + */
> > + if (likely(put_page_testzero(page))) {
> > + arch_clear_hugepage_flags(page);
> > + enqueue_huge_page(h, page);
> > + }
> > +
> > + return -ENOMEM;
> > + }
> > +
> > for (i = 0; i < pages_per_huge_page(h);
> > i++, subpage = mem_map_next(subpage, page, i)) {
> > subpage->flags &= ~(1 << PG_locked | 1 << PG_error |
> > @@ -1321,22 +1364,18 @@ static void update_and_free_page(struct hstate *h, struct page *page)
> > 1 << PG_active | 1 << PG_private |
> > 1 << PG_writeback);
> > }
> > - VM_BUG_ON_PAGE(hugetlb_cgroup_from_page(page), page);
> > - VM_BUG_ON_PAGE(hugetlb_cgroup_from_page_rsvd(page), page);
> > - set_compound_page_dtor(page, NULL_COMPOUND_DTOR);
> > - set_page_refcounted(page);
> > +
> > if (hstate_is_gigantic(h)) {
> > - /*
> > - * Temporarily drop the hugetlb_lock, because
> > - * we might block in free_gigantic_page().
> > - */
> > - spin_unlock(&hugetlb_lock);
> > destroy_compound_gigantic_page(page, huge_page_order(h));
> > free_gigantic_page(page, huge_page_order(h));
> > - spin_lock(&hugetlb_lock);
> > } else {
> > __free_pages(page, huge_page_order(h));
> > }
> > +
> > + if (free_vmemmap_pages_per_hpage(h) || hstate_is_gigantic(h))
> > + spin_lock(&hugetlb_lock);
> > +
> > + return 0;
> > }
> >
> > struct hstate *size_to_hstate(unsigned long size)
> > @@ -1404,9 +1443,9 @@ static void __free_huge_page(struct page *page)
> > } else if (h->surplus_huge_pages_node[nid]) {
> > /* remove the page from active list */
> > list_del(&page->lru);
> > - update_and_free_page(h, page);
> > h->surplus_huge_pages--;
> > h->surplus_huge_pages_node[nid]--;
> > + update_and_free_page(h, page);
> > } else {
> > arch_clear_hugepage_flags(page);
> > enqueue_huge_page(h, page);
> > @@ -1447,7 +1486,7 @@ void free_huge_page(struct page *page)
> > /*
> > * Defer freeing if in non-task context to avoid hugetlb_lock deadlock.
> > */
> > - if (!in_task()) {
> > + if (!in_atomic()) {
> > /*
> > * Only call schedule_work() if hpage_freelist is previously
> > * empty. Otherwise, schedule_work() had been called but the
> > @@ -1699,8 +1738,7 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed,
> > h->surplus_huge_pages--;
> > h->surplus_huge_pages_node[node]--;
> > }
> > - update_and_free_page(h, page);
> > - ret = 1;
> > + ret = !update_and_free_page(h, page);
> > break;
> > }
> > }
> > @@ -1713,10 +1751,14 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed,
> > * nothing for in-use hugepages and non-hugepages.
> > * This function returns values like below:
> > *
> > - * -EBUSY: failed to dissolved free hugepages or the hugepage is in-use
> > - * (allocated or reserved.)
> > - * 0: successfully dissolved free hugepages or the page is not a
> > - * hugepage (considered as already dissolved)
> > + * -ENOMEM: failed to allocate vmemmap pages to free the freed hugepages
> > + * when the system is under memory pressure and the feature of
> > + * freeing unused vmemmap pages associated with each hugetlb page
> > + * is enabled.
> > + * -EBUSY: failed to dissolved free hugepages or the hugepage is in-use
> > + * (allocated or reserved.)
> > + * 0: successfully dissolved free hugepages or the page is not a
> > + * hugepage (considered as already dissolved)
> > */
> > int dissolve_free_huge_page(struct page *page)
> > {
> > @@ -1771,8 +1813,12 @@ int dissolve_free_huge_page(struct page *page)
> > h->free_huge_pages--;
> > h->free_huge_pages_node[nid]--;
> > h->max_huge_pages--;
> > - update_and_free_page(h, head);
> > - rc = 0;
> > + rc = update_and_free_page(h, head);
> > + if (rc) {
> > + h->surplus_huge_pages--;
> > + h->surplus_huge_pages_node[nid]--;
> > + h->max_huge_pages++;
> > + }
> > }
> > out:
> > spin_unlock(&hugetlb_lock);
> > diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> > index 0209b736e0b4..f7ab3d99250a 100644
> > --- a/mm/hugetlb_vmemmap.c
> > +++ b/mm/hugetlb_vmemmap.c
> > @@ -181,21 +181,31 @@
> > #define RESERVE_VMEMMAP_NR 2U
> > #define RESERVE_VMEMMAP_SIZE (RESERVE_VMEMMAP_NR << PAGE_SHIFT)
> >
> > -/*
> > - * How many vmemmap pages associated with a HugeTLB page that can be freed
> > - * to the buddy allocator.
> > - *
> > - * Todo: Returns zero for now, which means the feature is disabled. We will
> > - * enable it once all the infrastructure is there.
> > - */
> > -static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
> > +static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h)
> > {
> > - return 0;
> > + return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT;
> > }
> >
> > -static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h)
> > +int alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
> > {
> > - return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT;
> > + unsigned long vmemmap_addr = (unsigned long)head;
> > + unsigned long vmemmap_end, vmemmap_reuse;
> > +
> > + if (!free_vmemmap_pages_per_hpage(h))
> > + return 0;
> > +
> > + vmemmap_addr += RESERVE_VMEMMAP_SIZE;
> > + vmemmap_end = vmemmap_addr + free_vmemmap_pages_size_per_hpage(h);
> > + vmemmap_reuse = vmemmap_addr - PAGE_SIZE;
>
> This is where I think some optimization is possible, once we are done with
> vmemmap_end calculation, we can use 6 pages (for 2MiB huge page) as pages
> for struct page. Is there a reason to not do so?

If you mean that we reuse part of a huge page as vmemmap while
freeing. You can look at the discussion here.

https://patchwork.kernel.org/project/linux-mm/patch/[email protected]/

Thanks.
>
> Balbir

2021-03-07 08:29:17

by Balbir Singh

Subject: Re: [PATCH v17 5/9] mm: hugetlb: set the PageHWPoison to the raw error page

On Thu, Feb 25, 2021 at 09:21:26PM +0800, Muchun Song wrote:
> Because we reuse the first tail vmemmap page frame and remap it
> with read-only, we cannot set the PageHWPosion on some tail pages.
> So we can use the head[4].private (There are at least 128 struct
> page structures associated with the optimized HugeTLB page, so
> using head[4].private is safe) to record the real error page index
> and set the raw error page PageHWPoison later.
>

Does the hardcoding of 4 come from HUGETLB_CGROUP_MIN_ORDER, if so
do we need to hardcode 4? Also, I am not sure about the comment
on safety and 128 struct pages

Balbir

2021-03-07 08:43:17

by Muchun Song

Subject: Re: [External] Re: [PATCH v17 5/9] mm: hugetlb: set the PageHWPoison to the raw error page

On Sun, Mar 7, 2021 at 4:19 PM Balbir Singh <[email protected]> wrote:
>
> On Thu, Feb 25, 2021 at 09:21:26PM +0800, Muchun Song wrote:
> > Because we reuse the first tail vmemmap page frame and remap it
> > with read-only, we cannot set the PageHWPosion on some tail pages.
> > So we can use the head[4].private (There are at least 128 struct
> > page structures associated with the optimized HugeTLB page, so
> > using head[4].private is safe) to record the real error page index
> > and set the raw error page PageHWPoison later.
> >
>
> Does the hardcoding of 4 come from HUGETLB_CGROUP_MIN_ORDER, if so

Yes.

> do we need to hardcode 4? Also, I am not sure about the comment
> on safety and 128 struct pages

We can set head[4].private only if free_vmemmap_pages_per_hpage(h)
returns true. In this case, there are 128 struct page structures (we reserve
2 pages as vmemmap pages, so 2 * 4KB / sizeof(struct page) == 128) that
can be used. Instead of hardcode, I introduce another patch to make the
code more readable. Please refer to patch #8 in this series.

Thanks.

>
> Balbir
>