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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 commitf27ce0e140
("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:f27ce0e140
("page_alloc: consider highatomic reserve in watermark fast") Signed-off-by: Jaewon Kim <jaewon31.kim@samsung.com> Reported-by: GyeongHwan Hong <gh21.hong@samsung.com> Acked-by: Mel Gorman <mgorman@techsingularity.net> Cc: Minchan Kim <minchan@kernel.org> Cc: Baoquan He <bhe@redhat.com> Cc: Vlastimil Babka <vbabka@suse.cz> Cc: Johannes Weiner <hannes@cmpxchg.org> Cc: Michal Hocko <mhocko@kernel.org> Cc: Yong-Taek Lee <ytk.lee@samsung.com> Cc: <stable@vger.kerenl.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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1 changed files with 8 additions and 4 deletions
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@ -3968,11 +3968,15 @@ static inline bool zone_watermark_fast(struct zone *z, unsigned int order,
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* need to be calculated.
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* need to be calculated.
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*/
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*/
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if (!order) {
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if (!order) {
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long fast_free;
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long usable_free;
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long reserved;
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fast_free = free_pages;
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usable_free = free_pages;
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fast_free -= __zone_watermark_unusable_free(z, 0, alloc_flags);
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reserved = __zone_watermark_unusable_free(z, 0, alloc_flags);
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if (fast_free > mark + z->lowmem_reserve[highest_zoneidx])
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/* reserved may over estimate high-atomic reserves. */
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usable_free -= min(usable_free, reserved);
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if (usable_free > mark + z->lowmem_reserve[highest_zoneidx])
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return true;
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return true;
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}
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}
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