From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx109.postini.com [74.125.245.109]) by kanga.kvack.org (Postfix) with SMTP id BA20B6B0031 for ; Wed, 11 Sep 2013 11:41:01 -0400 (EDT) Received: by mail-pb0-f47.google.com with SMTP id rr4so9192413pbb.6 for ; Wed, 11 Sep 2013 08:41:01 -0700 (PDT) Date: Wed, 11 Sep 2013 08:40:57 -0700 From: Anton Vorontsov Subject: Re: [PATCH] vmpressure: fix divide-by-0 in vmpressure_work_fn Message-ID: <20130911154057.GA16765@teo> References: <20130909110847.GB18056@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20130909110847.GB18056@dhcp22.suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: Hugh Dickins , Andrew Morton , David Rientjes , linux-kernel@vger.kernel.org, linux-mm@kvack.org 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 or moving the work into its own WQ_NON_REENTRANT queue. Anton -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org