Re: [PATCH] vmpressure: fix divide-by-0 in vmpressure_work_fn

[Michal Hocko]

On Wed 11-09-13 08:40:57, Anton Vorontsov wrote:
> 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

The patch below. I find it little bit nicer than Hugh's original one
because having the two checks sounds more confusing.
What do you think Hugh, Anton?

> or moving the work into its own WQ_NON_REENTRANT queue.

That sounds like an overkill.

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