linux-kernel - Re: [PATCH] vmpressure: fix divide-by-0 in vmpressure_work

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Date:	Wed, 11 Sep 2013 08:40:57 -0700
From:	Anton Vorontsov <anton@...msg.org>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] vmpressure: fix divide-by-0 in vmpressure_work_fn

On Mon, Sep 09, 2013 at 01:08:47PM +0200, Michal Hocko wrote:
> On Fri 06-09-13 22:59:16, Hugh Dickins wrote:
> > Hit divide-by-0 in vmpressure_work_fn(): checking vmpr->scanned before
> > taking the lock is not enough, we must check scanned afterwards too.
> 
> As vmpressure_work_fn seems the be the only place where we set scanned
> to 0 (except for the rare occasion when scanned overflows which
> would be really surprising) then the only possible way would be two
> vmpressure_work_fn racing over the same work item. system_wq is
> !WQ_NON_REENTRANT so one work item might be processed by multiple
> workers on different CPUs. This means that the vmpr->scanned check in
> the beginning of vmpressure_work_fn is inherently racy.
> 
> Hugh's patch fixes the issue obviously but doesn't it make more sense to
> move the initial vmpr->scanned check under the lock instead?
> 
> Anton, what was the initial motivation for the out of the lock
> check? Does it really optimize anything?

Thanks a lot for the explanation.

Answering your question: the idea was to minimize the lock section, but the
section is quite small anyway so I doubt that it makes any difference (during
development I could not measure any effect of vmpressure() calls in my system,
though the system itself was quite small).

I am happy with moving the check under the lock or moving the work into
its own WQ_NON_REENTRANT queue.

Anton
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