On 10/29/24 01:58, Andrew Morton wrote:
The patch titled Subject: mm/page_alloc: keep track of free highatomic has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-page_alloc-keep-track-of-free-highatomic.patch
In case of mm-hotfixes, the cc stable for 6.12 is unnecessary as it should make it there directly. Perhaps it's the best way, yeah.
Thanks, Vlastimil
This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...
This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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From: Yu Zhao yuzhao@google.com Subject: mm/page_alloc: keep track of free highatomic Date: Mon, 28 Oct 2024 12:26:53 -0600
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.
Link: https://lkml.kernel.org/r/20241028182653.3420139-1-yuzhao@google.com 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 Cc: stable@vger.kernel.org [6.12+] Signed-off-by: Andrew Morton akpm@linux-foundation.org
include/linux/mmzone.h | 1 + mm/page_alloc.c | 10 +++++++--- 2 files changed, 8 insertions(+), 3 deletions(-)
--- a/include/linux/mmzone.h~mm-page_alloc-keep-track-of-free-highatomic +++ a/include/linux/mmzone.h @@ -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 --- a/mm/page_alloc.c~mm-page_alloc-keep-track-of-free-highatomic +++ a/mm/page_alloc.c @@ -635,6 +635,8 @@ compaction_capture(struct capture_contro 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(str if (is_migrate_cma(migratetype)) __mod_zone_page_state(zone, NR_FREE_CMA_PAGES, nr_pages);
- if (is_migrate_highatomic(migratetype))
WRITE_ONCE(zone->nr_free_highatomic, zone->nr_free_highatomic + nr_pages);} /* Used for pages not on another list */ @@ -3081,11 +3086,10 @@ static inline long __zone_watermark_unus /* * 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.
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 */ _
Patches currently in -mm which might be from yuzhao@google.com are
mm-allow-set-clear-page_type-again.patch mm-multi-gen-lru-remove-mm_leaf_old-and-mm_nonleaf_total-stats.patch mm-multi-gen-lru-use-pteppmdp_clear_young_notify.patch mm-page_alloc-keep-track-of-free-highatomic.patch