| Message ID | 20220725012843.17115-1-jaewon31.kim@samsung.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | page_alloc: fix invalid watemark check on a negative value | expand |
On Mon, Jul 25, 2022 at 10:28:43AM +0900, Jaewon Kim 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. > > To handle the negative value, get the value as long type like > __zone_watermark_ok does. > > Reported-by: GyeongHwan Hong <gh21.hong@samsung.com> > Signed-off-by: Jaewon Kim <jaewon31.kim@samsung.com> Add Fixes: f27ce0e14088 ("page_alloc: consider highatomic reserve in watermark fast") The fix is fine as-is but it's not immediately obvious why this can wrap negative as it depends on an implementation detail of __zone_watermark_unusable_free. The variable copy just to change the sign could get accidentally "fixed" later as a micro-optimisation (same if the type of mark was changed) so maybe leave a comment like /* unusable may over-estimate high-atomic reserves */ Otherwise Acked-by: Mel Gorman <mgorman@techsingularity.net> The problem could also be made explicit with something like below. I know you are copying the logic of __zone_watermark_ok but I don't think min can go negative there. diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 934d1b5a5449..f8f50a2aa43e 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -4048,11 +4048,15 @@ 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; }
>On Mon, Jul 25, 2022 at 10:28:43AM +0900, Jaewon Kim 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. >> >> To handle the negative value, get the value as long type like >> __zone_watermark_ok does. >> >> Reported-by: GyeongHwan Hong <gh21.hong@samsung.com> >> Signed-off-by: Jaewon Kim <jaewon31.kim@samsung.com> > >Add > >Fixes: f27ce0e14088 ("page_alloc: consider highatomic reserve in watermark fast") I will add the Fixes. > >The fix is fine as-is but it's not immediately obvious why this >can wrap negative as it depends on an implementation detail of >__zone_watermark_unusable_free. The variable copy just to change the sign >could get accidentally "fixed" later as a micro-optimisation (same if the >type of mark was changed) so maybe leave a comment like > > /* unusable may over-estimate high-atomic reserves */ > >Otherwise > >Acked-by: Mel Gorman <mgorman@techsingularity.net> Thank you for your Ack Yes leaving comment will be helpful. Actually let me take your patch. I think this but is obvious and fix is sipmle, I can resubmit right away. > >The problem could also be made explicit with something like below. I know >you are copying the logic of __zone_watermark_ok but I don't think min >can go negative there. The min in __zone_watermark_ok is positive because mark is always unsigned. But I think free_pages in __zone_watermark_ok can go negative because of the same reason. > >diff --git a/mm/page_alloc.c b/mm/page_alloc.c >index 934d1b5a5449..f8f50a2aa43e 100644 >--- a/mm/page_alloc.c >+++ b/mm/page_alloc.c >@@ -4048,11 +4048,15 @@ 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; > } > >-- >Mel Gorman >SUSE Labs
diff --git a/mm/page_alloc.c b/mm/page_alloc.c index e008a3df0485..cf667fae132e 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -3968,11 +3968,12 @@ static inline bool zone_watermark_fast(struct zone *z, unsigned int order, * need to be calculated. */ if (!order) { + long min = mark; long fast_free; fast_free = free_pages; fast_free -= __zone_watermark_unusable_free(z, 0, alloc_flags); - if (fast_free > mark + z->lowmem_reserve[highest_zoneidx]) + if (fast_free > min + 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. To handle the negative value, get the value as long type like __zone_watermark_ok does. Reported-by: GyeongHwan Hong <gh21.hong@samsung.com> Signed-off-by: Jaewon Kim <jaewon31.kim@samsung.com> --- mm/page_alloc.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)