RE: [PATCH v2] page_alloc: fix invalid watemark check on a negative value

[Jaewon Kim]

> 
> 
>--------- Original Message ---------
>Sender : Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>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@xxxxxxxxxxx> 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.