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Date:   Wed, 18 Aug 2021 10:15:24 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Roman Gushchin <guro@fb.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Leon Yang <lnyng@fb.com>, Chris Down <chris@chrisdown.name>,
        Michal Hocko <mhocko@suse.com>, linux-mm@kvack.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@fb.com
Subject: Re: [PATCH] mm: memcontrol: fix occasional OOMs due to proportional
 memory.low reclaim
Message-ID: <YR0V/KhKSYZs+ksn@cmpxchg.org>
References: <20210817180506.220056-1-hannes@cmpxchg.org>
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On Tue, Aug 17, 2021 at 12:45:18PM -0700, Roman Gushchin wrote:
> On Tue, Aug 17, 2021 at 02:05:06PM -0400, Johannes Weiner wrote:
> > We've noticed occasional OOM killing when memory.low settings are in
> > effect for cgroups. This is unexpected and undesirable as memory.low
> > is supposed to express non-OOMing memory priorities between cgroups.
> > 
> > The reason for this is proportional memory.low reclaim. When cgroups
> > are below their memory.low threshold, reclaim passes them over in the
> > first round, and then retries if it couldn't find pages anywhere else.
> > But when cgroups are slighly above their memory.low setting, page scan
> > force is scaled down and diminished in proportion to the overage, to
> > the point where it can cause reclaim to fail as well - only in that
> > case we currently don't retry, and instead trigger OOM.
> > 
> > To fix this, hook proportional reclaim into the same retry logic we
> > have in place for when cgroups are skipped entirely. This way if
> > reclaim fails and some cgroups were scanned with dimished pressure,
> > we'll try another full-force cycle before giving up and OOMing.
> > 
> > Reported-by: Leon Yang <lnyng@fb.com>
> > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> 
> Acked-by: Roman Gushchin <guro@fb.com>

Thank you.

> I guess it's a stable material, so maybe adding:
> Fixes: 9783aa9917f8 ("mm, memcg: proportional memory.{low,min} reclaim")

Yes, that Fixes makes sense. Plus:

Cc: <stable@vger.kernel.org> # 5.4+

I initially didn't tag it because the issue is over two years old and
we've had no other reports of this. But thinking about it, it's
probably more a lack of users rather than severity. At FB we only
noticed with a recent rollout of memory_recursiveprot
(8a931f801340c2be10552c7b5622d5f4852f3a36) because we didn't have
working memory.low configurations before that. But now that we do
notice, it's a problem worth fixing. So yes, stable makes sense.

Thanks.