From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx112.postini.com [74.125.245.112]) by kanga.kvack.org (Postfix) with SMTP id 1D9416B0031 for ; Wed, 11 Sep 2013 12:03:59 -0400 (EDT) Date: Wed, 11 Sep 2013 18:03:57 +0200 From: Michal Hocko Subject: Re: [PATCH] vmpressure: fix divide-by-0 in vmpressure_work_fn Message-ID: <20130911160357.GA32273@dhcp22.suse.cz> References: <20130909110847.GB18056@dhcp22.suse.cz> <20130911154057.GA16765@teo> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130911154057.GA16765@teo> Sender: owner-linux-mm@kvack.org List-ID: To: Hugh Dickins , Anton Vorontsov Cc: Andrew Morton , David Rientjes , linux-kernel@vger.kernel.org, linux-mm@kvack.org 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. ---