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mm/page_alloc: keep track of free highatomic

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OOM kills due to vastly overestimated free highatomic reserves were
observed:

  ... invoked oom-killer: gfp_mask=0x100cca(GFP_HIGHUSER_MOVABLE), order=0 ...
  Node 0 Normal free:1482936kB boost:0kB min:410416kB low:739404kB high:1068392kB reserved_highatomic:1073152KB ...
  Node 0 Normal: 1292*4kB (ME) 1920*8kB (E) 383*16kB (UE) 220*32kB (ME) 340*64kB (E) 2155*128kB (UE) 3243*256kB (UE) 615*512kB (U) 1*1024kB (M) 0*2048kB 0*4096kB = 1477408kB

The second line above shows that the OOM kill was due to the following
condition:

  free (1482936kB) - reserved_highatomic (1073152kB) = 409784KB < min (410416kB)

And the third line shows there were no free pages in any
MIGRATE_HIGHATOMIC pageblocks, which otherwise would show up as type 'H'. 
Therefore __zone_watermark_unusable_free() underestimated the usable free
memory by over 1GB, which resulted in the unnecessary OOM kill above.

The comments in __zone_watermark_unusable_free() warns about the potential
risk, i.e.,

  If the caller does not have rights to reserves below the min
  watermark then subtract the high-atomic reserves. This will
  over-estimate the size of the atomic reserve but it avoids a search.

However, it is possible to keep track of free pages in reserved highatomic
pageblocks with a new per-zone counter nr_free_highatomic protected by the
zone lock, to avoid a search when calculating the usable free memory.  And
the cost would be minimal, i.e., simple arithmetics in the highatomic
alloc/free/move paths.

Note that since nr_free_highatomic can be relatively small, using a
per-cpu counter might cause too much drift and defeat its purpose, in
addition to the extra memory overhead.

Dependson e0932b6c1f ("mm: page_alloc: consolidate free page accounting") - see [1]

[akpm@linux-foundation.org: s/if/else if/, per Johannes, stealth whitespace tweak]
Link: https://lkml.kernel.org/r/20241028182653.3420139-1-yuzhao@google.com
Link: https://lkml.kernel.org/r/0d0ddb33-fcdc-43e2-801f-0c1df2031afb@suse.cz [1]
Fixes: 0aaa29a56e ("mm, page_alloc: reserve pageblocks for high-order atomic allocations on demand")
Signed-off-by: Yu Zhao <yuzhao@google.com>
Reported-by: Link Lin <linkl@google.com>
Acked-by: David Rientjes <rientjes@google.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Yu Zhao 2024-10-28 12:26:53 -06:00 committed by Andrew Morton
parent 5de195060b
commit c928807f6f
2 changed files with 8 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
include/linux/mmzone.h
View File

@ -823,6 +823,7 @@ struct zone {
unsigned long watermark_boost;
unsigned long nr_reserved_highatomic;
unsigned long nr_free_highatomic;
/*
* We don't know if the memory that we're going to allocate will be

10
mm/page_alloc.c
View File

@ -635,6 +635,8 @@ compaction_capture(struct capture_control *capc, struct page *page,
static inline void account_freepages(struct zone *zone, int nr_pages,
int migratetype)
{
lockdep_assert_held(&zone->lock);
if (is_migrate_isolate(migratetype))
return;
@ -642,6 +644,9 @@ static inline void account_freepages(struct zone *zone, int nr_pages,
if (is_migrate_cma(migratetype))
__mod_zone_page_state(zone, NR_FREE_CMA_PAGES, nr_pages);
else if (is_migrate_highatomic(migratetype))
WRITE_ONCE(zone->nr_free_highatomic,
zone->nr_free_highatomic + nr_pages);
}
/* Used for pages not on another list */
@ -3079,11 +3084,10 @@ static inline long __zone_watermark_unusable_free(struct zone *z,
/*
* If the caller does not have rights to reserves below the min
* watermark then subtract the high-atomic reserves. This will
* over-estimate the size of the atomic reserve but it avoids a search.
* watermark then subtract the free pages reserved for highatomic.
*/
if (likely(!(alloc_flags & ALLOC_RESERVES)))
unusable_free += z->nr_reserved_highatomic;
unusable_free += READ_ONCE(z->nr_free_highatomic);
#ifdef CONFIG_CMA
/* If allocation can't use CMA areas don't use free CMA pages */