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

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Date:	Mon, 9 Sep 2013 13:08:47 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Anton Vorontsov <anton.vorontsov@...aro.org>,
	Hugh Dickins <hughd@...gle.com>
Cc:	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 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@...gle.com>
> Cc: stable@...r.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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