This is a note to let you know that I've just added the patch titled page_alloc: fix invalid watermark check on a negative value to the 5.18-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: page_alloc-fix-invalid-watermark-check-on-a-negative-value.patch and it can be found in the queue-5.18 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From 9282012fc0aa248b77a69f5eb802b67c5a16bb13 Mon Sep 17 00:00:00 2001 From: Jaewon Kim <jaewon31.kim@xxxxxxxxxxx> Date: Mon, 25 Jul 2022 18:52:12 +0900 Subject: page_alloc: fix invalid watermark check on a negative value From: Jaewon Kim <jaewon31.kim@xxxxxxxxxxx> commit 9282012fc0aa248b77a69f5eb802b67c5a16bb13 upstream. 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> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- mm/page_alloc.c | 12 ++++++++---- 1 file changed, 8 insertions(+), 4 deletions(-) --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -3953,11 +3953,15 @@ 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 stable-queue which might be from jaewon31.kim@xxxxxxxxxxx are queue-5.18/page_alloc-fix-invalid-watermark-check-on-a-negative-value.patch