linux-kernel - Re: [PATCH] mm,vmscan: fix divide by zero in get_scan

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Message-ID: <YSkSnEYfrWMMgJgQ@carbon.dhcp.thefacebook.com>
Date:   Fri, 27 Aug 2021 09:28:12 -0700
From:   Roman Gushchin <guro@...com>
To:     Rik van Riel <riel@...riel.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, <kernel-team@...com>,
        <stable@...nel.org>, Michal Hocko <mhocko@...e.com>,
        Chris Down <chris@...isdown.name>,
        Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH] mm,vmscan: fix divide by zero in get_scan_count

On Thu, Aug 26, 2021 at 10:01:49PM -0400, 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.
> 
> Fixes: f56ce412a59d ("mm: memcontrol: fix occasional OOMs due to proportional memory.low reclaim")
> Signed-off-by: Rik van Riel <riel@...riel.com>

It looks like we discover a new divide-by-zero corner case after every
functional change to the memory protection code :)

Acked-by: Roman Gushchin <guro@...com>

Thanks, Rik!