The patch titled
Subject: page_alloc: fix invalid watemark check on a negative value
has been added to the -mm mm-hotfixes-unstable branch. Its filename is
page_alloc-fix-invalid-watemark-check-on-a-negative-value.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/page_alloc-fix-invalid-watemark-check-on-a-negative-value.patch
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: Jaewon Kim <jaewon31.kim@xxxxxxxxxxx>
Subject: page_alloc: fix invalid watemark check on a negative value
Date: Mon, 25 Jul 2022 18:52:12 +0900
There was a report that a task is waiting at the
throttle_direct_reclaim. The pgscan_direct_throttle in vmstat was
increasing.
This is a bug where zone_watermark_fast returns true even when the free
is very low. The commit f27ce0e14088 ("page_alloc: consider highatomic
reserve in watermark fast") changed the watermark fast to consider
highatomic reserve. But it did not handle a negative value case which
can be happened when reserved_highatomic pageblock is bigger than the
actual free.
If watermark is considered as ok for the negative value, allocating
contexts for order-0 will consume all free pages without direct reclaim,
and finally free page may become depleted except highatomic free.
Then allocating contexts may fall into throttle_direct_reclaim. This
symptom may easily happen in a system where wmark min is low and other
reclaimers like kswapd does not make free pages quickly.
Handle the negative case by using MIN.
Link: https://lkml.kernel.org/r/20220725095212.25388-1-jaewon31.kim@xxxxxxxxxxx
Fixes: f27ce0e14088 ("page_alloc: consider highatomic reserve in watermark fast")
Signed-off-by: Jaewon Kim <jaewon31.kim@xxxxxxxxxxx>
Reported-by: GyeongHwan Hong <gh21.hong@xxxxxxxxxxx>
Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Baoquan He <bhe@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Yong-Taek Lee <ytk.lee@xxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/page_alloc.c | 12 ++++++++----
1 file changed, 8 insertions(+), 4 deletions(-)
--- a/mm/page_alloc.c~page_alloc-fix-invalid-watemark-check-on-a-negative-value
+++ a/mm/page_alloc.c
@@ -3968,11 +3968,15 @@ static inline bool zone_watermark_fast(s
* need to be calculated.
*/
if (!order) {
- long fast_free;
+ long usable_free;
+ long reserved;
- fast_free = free_pages;
- fast_free -= __zone_watermark_unusable_free(z, 0, alloc_flags);
- if (fast_free > mark + z->lowmem_reserve[highest_zoneidx])
+ usable_free = free_pages;
+ reserved = __zone_watermark_unusable_free(z, 0, alloc_flags);
+
+ /* reserved may over estimate high-atomic reserves. */
+ usable_free -= min(usable_free, reserved);
+ if (usable_free > mark + z->lowmem_reserve[highest_zoneidx])
return true;
}
_
Patches currently in -mm which might be from jaewon31.kim@xxxxxxxxxxx are
page_alloc-fix-invalid-watemark-check-on-a-negative-value.patch
