In the booting phase if CONFIG_DEFERRED_STRUCT_PAGE_INIT is set,
we have following callchain:
start_kernel
...
mm_init
mem_init
memblock_free_all
reset_all_zones_managed_pages
free_low_memory_core_early
...
buffer_init
nr_free_buffer_pages
zone->managed_pages
...
rest_init
kernel_init
kernel_init_freeable
page_alloc_init_late
kthread_run(deferred_init_memmap, NODE_DATA(nid), "pgdatinit%d", nid);
wait_for_completion(&pgdat_init_all_done_comp);
...
files_maxfiles_init
It's clear that buffer_init depends on zone->managed_pages, but it's reset
in reset_all_zones_managed_pages after that pages are readded into
zone->managed_pages, but when buffer_init runs this process is half done
and most of them will finally be added till deferred_init_memmap done.
In large memory couting of nr_free_buffer_pages drifts too much, also
drifting from kernels to kernels on same hardware.
Fix is simple, it delays buffer_init run till deferred_init_memmap all done.
But as corrected by this patch, max_buffer_heads becomes very large,
the value is roughly as many as 4 times of totalram_pages, formula:
max_buffer_heads = nrpages * (10%) * (PAGE_SIZE / sizeof(struct buffer_head));
Say in a 64GB memory box we have 16777216 pages, then max_buffer_heads
turns out to be roughly 67,108,864.
In common cases, should a buffer_head be mapped to one page/block(4KB)?
So max_buffer_heads never exceeds totalram_pages.
IMO it's likely to make buffer_heads_over_limit bool value alwasy false,
then make codes 'if (buffer_heads_over_limit)' test in vmscan unnecessary.
Correct me if it's not true.
So this patch will change the original behavior related to
buffer_heads_over_limit in vmscan since we used a half done value
of zone->managed_pages before, or should we use a smaller factor(<10%) in
previous formula.
Signed-off-by: Lin Feng <[email protected]>
---
init/main.c | 2 --
mm/page_alloc.c | 3 +++
2 files changed, 3 insertions(+), 2 deletions(-)
diff --git a/init/main.c b/init/main.c
index 20baced721ad..a3f7c3416286 100644
--- a/init/main.c
+++ b/init/main.c
@@ -58,7 +58,6 @@
#include <linux/rmap.h>
#include <linux/mempolicy.h>
#include <linux/key.h>
-#include <linux/buffer_head.h>
#include <linux/page_ext.h>
#include <linux/debug_locks.h>
#include <linux/debugobjects.h>
@@ -1034,7 +1033,6 @@ asmlinkage __visible void __init __no_sanitize_address start_kernel(void)
fork_init();
proc_caches_init();
uts_ns_init();
- buffer_init();
key_init();
security_init();
dbg_late_init();
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index eaa227a479e4..2931d706fb52 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -70,6 +70,7 @@
#include <linux/psi.h>
#include <linux/padata.h>
#include <linux/khugepaged.h>
+#include <linux/buffer_head.h>
#include <asm/sections.h>
#include <asm/tlbflush.h>
@@ -2103,6 +2104,8 @@ void __init page_alloc_init_late(void)
files_maxfiles_init();
#endif
+ buffer_init();
+
/* Discard memblock private memory */
memblock_discard();
--
2.25.4
On Mon, 23 Nov 2020 19:05:00 +0800 Lin Feng <[email protected]> wrote:
> In the booting phase if CONFIG_DEFERRED_STRUCT_PAGE_INIT is set,
> we have following callchain:
>
> start_kernel
> ...
> mm_init
> mem_init
> memblock_free_all
> reset_all_zones_managed_pages
> free_low_memory_core_early
> ...
> buffer_init
> nr_free_buffer_pages
> zone->managed_pages
> ...
> rest_init
> kernel_init
> kernel_init_freeable
> page_alloc_init_late
> kthread_run(deferred_init_memmap, NODE_DATA(nid), "pgdatinit%d", nid);
> wait_for_completion(&pgdat_init_all_done_comp);
> ...
> files_maxfiles_init
>
> It's clear that buffer_init depends on zone->managed_pages, but it's reset
> in reset_all_zones_managed_pages after that pages are readded into
> zone->managed_pages, but when buffer_init runs this process is half done
> and most of them will finally be added till deferred_init_memmap done.
> In large memory couting of nr_free_buffer_pages drifts too much, also
> drifting from kernels to kernels on same hardware.
>
> Fix is simple, it delays buffer_init run till deferred_init_memmap all done.
>
> But as corrected by this patch, max_buffer_heads becomes very large,
> the value is roughly as many as 4 times of totalram_pages, formula:
> max_buffer_heads = nrpages * (10%) * (PAGE_SIZE / sizeof(struct buffer_head));
>
> Say in a 64GB memory box we have 16777216 pages, then max_buffer_heads
> turns out to be roughly 67,108,864.
> In common cases, should a buffer_head be mapped to one page/block(4KB)?
> So max_buffer_heads never exceeds totalram_pages.
> IMO it's likely to make buffer_heads_over_limit bool value alwasy false,
> then make codes 'if (buffer_heads_over_limit)' test in vmscan unnecessary.
> Correct me if it's not true.
I agree - seems that on such a system we'll allow enough buffer_heads
to manage about 250GB worth of pagecache, for a 4kb filesystem
blocksize.
Perhaps this code is all a remnant of highmem systems, where
ZONE_NORMAL is considerably smaller than ZONE_HIGHMEM, and we don't
want to be consuming all of ZONE_NORMAL for highmem-attached
buffer_heads.
I'm not sure that it's all very harmful - we don't *need* to be
trimming away at the buffer_heads on a 64GB 4-bit system so the code is
really only functional on highmem machines. And as far as I know, it
works OK on such machines.
On 11/23/20 12:05 PM, Lin Feng wrote:
> In the booting phase if CONFIG_DEFERRED_STRUCT_PAGE_INIT is set,
> we have following callchain:
>
> start_kernel
> ...
> mm_init
> mem_init
> memblock_free_all
> reset_all_zones_managed_pages
> free_low_memory_core_early
> ...
> buffer_init
> nr_free_buffer_pages
> zone->managed_pages
> ...
> rest_init
> kernel_init
> kernel_init_freeable
> page_alloc_init_late
> kthread_run(deferred_init_memmap, NODE_DATA(nid), "pgdatinit%d", nid);
> wait_for_completion(&pgdat_init_all_done_comp);
> ...
> files_maxfiles_init
>
> It's clear that buffer_init depends on zone->managed_pages, but it's reset
> in reset_all_zones_managed_pages after that pages are readded into
> zone->managed_pages, but when buffer_init runs this process is half done
> and most of them will finally be added till deferred_init_memmap done.
> In large memory couting of nr_free_buffer_pages drifts too much, also
> drifting from kernels to kernels on same hardware.
>
> Fix is simple, it delays buffer_init run till deferred_init_memmap all done.
Hmm nobody should need bh_cachep to allocate buffer heads until then, right.
> But as corrected by this patch, max_buffer_heads becomes very large,
> the value is roughly as many as 4 times of totalram_pages, formula:
> max_buffer_heads = nrpages * (10%) * (PAGE_SIZE / sizeof(struct buffer_head));
>
> Say in a 64GB memory box we have 16777216 pages, then max_buffer_heads
> turns out to be roughly 67,108,864.
> In common cases, should a buffer_head be mapped to one page/block(4KB)?
> So max_buffer_heads never exceeds totalram_pages.
> IMO it's likely to make buffer_heads_over_limit bool value alwasy false,
> then make codes 'if (buffer_heads_over_limit)' test in vmscan unnecessary.
> Correct me if it's not true.
Maybe we could compile that out with CONFIG_HIGHMEM then?
> So this patch will change the original behavior related to
> buffer_heads_over_limit in vmscan since we used a half done value
> of zone->managed_pages before, or should we use a smaller factor(<10%) in
> previous formula.
>
> Signed-off-by: Lin Feng <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
> ---
> init/main.c | 2 --
> mm/page_alloc.c | 3 +++
> 2 files changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/init/main.c b/init/main.c
> index 20baced721ad..a3f7c3416286 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -58,7 +58,6 @@
> #include <linux/rmap.h>
> #include <linux/mempolicy.h>
> #include <linux/key.h>
> -#include <linux/buffer_head.h>
> #include <linux/page_ext.h>
> #include <linux/debug_locks.h>
> #include <linux/debugobjects.h>
> @@ -1034,7 +1033,6 @@ asmlinkage __visible void __init __no_sanitize_address start_kernel(void)
> fork_init();
> proc_caches_init();
> uts_ns_init();
> - buffer_init();
> key_init();
> security_init();
> dbg_late_init();
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index eaa227a479e4..2931d706fb52 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -70,6 +70,7 @@
> #include <linux/psi.h>
> #include <linux/padata.h>
> #include <linux/khugepaged.h>
> +#include <linux/buffer_head.h>
>
> #include <asm/sections.h>
> #include <asm/tlbflush.h>
> @@ -2103,6 +2104,8 @@ void __init page_alloc_init_late(void)
> files_maxfiles_init();
> #endif
>
> + buffer_init();
> +
> /* Discard memblock private memory */
> memblock_discard();
>
>