From 9282012fc0aa248b77a69f5eb802b67c5a16bb13 Mon Sep 17 00:00:00 2001 From: Jaewon Kim Date: Mon, 25 Jul 2022 18:52:12 +0900 Subject: [PATCH] page_alloc: fix invalid watermark check on a negative value 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@samsung.com 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 --- mm/page_alloc.c | 12 ++++++++---- 1 file changed, 8 insertions(+), 4 deletions(-) diff --git a/mm/page_alloc.c b/mm/page_alloc.c index e008a3d..b5b14b7 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -3968,11 +3968,15 @@ static inline bool zone_watermark_fast(struct zone *z, unsigned int order, * 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; } -- 2.7.4