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Subject[PATCH 5.15 64/69] page_alloc: fix invalid watermark check on a negative value
From: Jaewon Kim <>

commit 9282012fc0aa248b77a69f5eb802b67c5a16bb13 upstream.

There was a report that a task is waiting at the
throttle_direct_reclaim. The pgscan_direct_throttle in vmstat was

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.

Fixes: f27ce0e14088 ("page_alloc: consider highatomic reserve in watermark fast")
Signed-off-by: Jaewon Kim <>
Reported-by: GyeongHwan Hong <>
Acked-by: Mel Gorman <>
Cc: Minchan Kim <>
Cc: Baoquan He <>
Cc: Vlastimil Babka <>
Cc: Johannes Weiner <>
Cc: Michal Hocko <>
Cc: Yong-Taek Lee <>
Cc: <>
Signed-off-by: Andrew Morton <>
Signed-off-by: Greg Kroah-Hartman <>
mm/page_alloc.c | 12 ++++++++----
1 file changed, 8 insertions(+), 4 deletions(-)

--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3928,11 +3928,15 @@ 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;

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