Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20210225132130.26451-1-songmuchun@bytedance.com>
 <20210225132130.26451-4-songmuchun@bytedance.com> <6a4a4fd8-d272-c034-f88c-cf372d8825a0@gmail.com>
In-Reply-To: <6a4a4fd8-d272-c034-f88c-cf372d8825a0@gmail.com>
From:   Muchun Song <songmuchun@bytedance.com>
Date:   Fri, 5 Mar 2021 12:41:30 +0800
Message-ID: <CAMZfGtXJsq5u4MaT1Qo=4zqdLE_2Pax5Y4x2nYe0r6RDMAEwkw@mail.gmail.com>
Subject: Re: [External] Re: [PATCH v17 3/9] mm: hugetlb: free the vmemmap
 pages associated with each HugeTLB page
To:     "Singh, Balbir" <bsingharora@gmail.com>,
        Oscar Salvador <osalvador@suse.de>,
        Mike Kravetz <mike.kravetz@oracle.com>
Cc:     Jonathan Corbet <corbet@lwn.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, bp@alien8.de, x86@kernel.org,
        hpa@zytor.com, dave.hansen@linux.intel.com, luto@kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>, paulmck@kernel.org,
        mchehab+huawei@kernel.org, pawan.kumar.gupta@linux.intel.com,
        Randy Dunlap <rdunlap@infradead.org>, oneukum@suse.com,
        anshuman.khandual@arm.com, jroedel@suse.de,
        Mina Almasry <almasrymina@google.com>,
        David Rientjes <rientjes@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        Michal Hocko <mhocko@suse.com>,
        "Song Bao Hua (Barry Song)" <song.bao.hua@hisilicon.com>,
        David Hildenbrand <david@redhat.com>,
        =?UTF-8?B?SE9SSUdVQ0hJIE5BT1lBKOWggOWPoyDnm7TkuZ8p?= 
        <naoya.horiguchi@nec.com>,
        Joao Martins <joao.m.martins@oracle.com>,
        Xiongchun duan <duanxiongchun@bytedance.com>,
        linux-doc@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Fri, Mar 5, 2021 at 7:50 AM Singh, Balbir <bsingharora@gmail.com> 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 <songmuchun@bytedance.com>
> > Reviewed-by: Oscar Salvador <osalvador@suse.de>
> > ---
> >  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 <songmuchun@bytedance.com>
> > + *
> > + * 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 <songmuchun@bytedance.com>
> > + */
> > +#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/20201217121303.13386-4-songmuchun@bytedance.com/

> 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