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Date:   Mon, 30 Aug 2021 13:33:10 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Rik van Riel <riel@surriel.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, kernel-team@fb.com,
        stable@kernel.org, Chris Down <chris@chrisdown.name>,
        Johannes Weiner <hannes@cmpxchg.org>
Subject: Re: [PATCH] mm,vmscan: fix divide by zero in get_scan_count
Message-ID: <YSzB9q1kVjKce7ly@dhcp22.suse.cz>
References: <20210826220149.058089c6@imladris.surriel.com>
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On Thu 26-08-21 22:01:49, Rik van Riel wrote:
> Changeset f56ce412a59d ("mm: memcontrol: fix occasional OOMs due to
> proportional memory.low reclaim") introduced a divide by zero corner
> case when oomd is being used in combination with cgroup memory.low
> protection.
> 
> When oomd decides to kill a cgroup, it will force the cgroup memory
> to be reclaimed after killing the tasks, by writing to the memory.max
> file for that cgroup, forcing the remaining page cache and reclaimable
> slab to be reclaimed down to zero.
> 
> Previously, on cgroups with some memory.low protection that would result
> in the memory being reclaimed down to the memory.low limit, or likely not
> at all, having the page cache reclaimed asynchronously later.
> 
> With f56ce412a59d the oomd write to memory.max tries to reclaim all the
> way down to zero, which may race with another reclaimer, to the point of
> ending up with the divide by zero below.
> 
> This patch implements the obvious fix.

I must be missing something but how can cgroup_size be ever 0 when it is
max(cgroup_size, protection) and protection != 0?
-- 
Michal Hocko
SUSE Labs