Messages in this thread | | | Date | Mon, 9 Sep 2013 13:08:47 +0200 | From | Michal Hocko <> | Subject | Re: [PATCH] vmpressure: fix divide-by-0 in vmpressure_work_fn |
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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?
> Signed-off-by: Hugh Dickins <hughd@google.com> > Cc: stable@vger.kernel.org > --- > > mm/vmpressure.c | 3 +++ > 1 file changed, 3 insertions(+) > > --- 3.11/mm/vmpressure.c 2013-09-02 13:46:10.000000000 -0700 > +++ linux/mm/vmpressure.c 2013-09-06 22:43:03.596003080 -0700 > @@ -187,6 +187,9 @@ static void vmpressure_work_fn(struct wo > vmpr->reclaimed = 0; > spin_unlock(&vmpr->sr_lock); > > + if (!scanned) > + return; > + > do { > if (vmpressure_event(vmpr, scanned, reclaimed)) > break;
-- Michal Hocko SUSE Labs
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