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

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Message-ID: <YSzf/3A6b0R3kR5v@dhcp22.suse.cz>
Date:   Mon, 30 Aug 2021 15:41:19 +0200
From:   Michal Hocko <mhocko@...e.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, Chris Down <chris@...isdown.name>,
        Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH] mm,vmscan: fix divide by zero in get_scan_count

On Mon 30-08-21 09:24:22, Rik van Riel wrote:
> On Mon, 2021-08-30 at 13:33 +0200, Michal Hocko wrote:
[...]
> > I must be missing something but how can cgroup_size be ever 0 when it
> > is
> > max(cgroup_size, protection) and protection != 0?
> 
> Going into the condition we use if (low || min), where
> it is possible for low > 0 && min == 0.
> 
> Inside the conditional, we can end up testing against
> min.

Dang, I was looking at the tree without f56ce412a59d7 applied. My bad!
Personally I would consider the following slightly easier to follow
			scan = lruvec_size - lruvec_size * protection /
				max(cgroup_size, 1);

The code is quite tricky already and if you asked me what kind of effect
cgroup_size + 1 have there I would just shrug shoulders...

Anyway your fix will prevent the reported problem and I cannot see any
obvious problem with it either so
Acked-by: Michal Hocko <mhocko@...e.com>
-- 
Michal Hocko
SUSE Labs