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@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
>
> 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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