linux-kernel - Re: [PATCH v2] page_alloc: fix invalid watemark check on a negative value

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Date:   Mon, 25 Jul 2022 11:37:07 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Jaewon Kim <jaewon31.kim@...sung.com>
Cc:     minchan@...nel.org, bhe@...hat.com, vbabka@...e.cz,
        mgorman@...hsingularity.net, hannes@...xchg.org, mhocko@...nel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        gh21.hong@...sung.com, ytk.lee@...sung.com, jaewon31.kim@...il.com
Subject: 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@...sung.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.