| Message ID | 20220725095212.25388-1-jaewon31.kim@samsung.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [v2] page_alloc: fix invalid watemark check on a negative value | expand |
On Mon, 25 Jul 2022 18:52:12 +0900 Jaewon Kim <jaewon31.kim@samsung.com> wrote: > 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. > Thanks, I added cc:stable to this.
> > >--------- Original Message --------- >Sender : Andrew Morton <akpm@linux-foundation.org> >Date : 2022-07-26 03:37 (GMT+9) >Title : Re: [PATCH v2] page_alloc: fix invalid watemark check on a negative value > >On Mon, 25 Jul 2022 18:52:12 +0900 Jaewon Kim <jaewon31.kim@samsung.com> wrote: > >> 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. >> > >Thanks, I added cc:stable to this. > Sorry for your inconvenience. Could you fix a typo by adding r to watermark on title - page_alloc: fix invalid watemark check on a negative value + page_alloc: fix invalid watermark check on a negative value I've appreciated it in advance.
diff --git a/mm/page_alloc.c b/mm/page_alloc.c index e008a3df0485..b5b14b78c4fd 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; }
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. Fixes: f27ce0e14088 ("page_alloc: consider highatomic reserve in watermark fast") Reported-by: GyeongHwan Hong <gh21.hong@samsung.com> Signed-off-by: Jaewon Kim <jaewon31.kim@samsung.com> Acked-by: Mel Gorman <mgorman@techsingularity.net> --- v2: use explicit code suggested by Mel Gorman v1: use signed min --- mm/page_alloc.c | 12 ++++++++---- 1 file changed, 8 insertions(+), 4 deletions(-)