| From: Jaewon Kim <jaewon31.kim@samsung.com> |
| Subject: page_alloc: fix invalid watermark 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@samsung.com |
| Fixes: f27ce0e14088 ("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> |
| --- |
| |
| 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; |
| } |
| |
| _ |
