From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pa0-f53.google.com (mail-pa0-f53.google.com [209.85.220.53]) by kanga.kvack.org (Postfix) with ESMTP id A00E56B0035 for ; Wed, 30 Apr 2014 16:13:56 -0400 (EDT) Received: by mail-pa0-f53.google.com with SMTP id ld10so2568039pab.12 for ; Wed, 30 Apr 2014 13:13:56 -0700 (PDT) Received: from mail.linuxfoundation.org (mail.linuxfoundation.org. [140.211.169.12]) by mx.google.com with ESMTP id wh9si18092904pac.418.2014.04.30.13.13.55 for ; Wed, 30 Apr 2014 13:13:55 -0700 (PDT) Date: Wed, 30 Apr 2014 13:13:53 -0700 From: Andrew Morton Subject: Re: [PATCH v4] mm,writeback: fix divide by zero in pos_ratio_polynom Message-Id: <20140430131353.fa9f49604ea39425bc93c24a@linux-foundation.org> In-Reply-To: <20140430160218.442863e0@cuia.bos.redhat.com> References: <20140429151910.53f740ef@annuminas.surriel.com> <5360C9E7.6010701@jp.fujitsu.com> <20140430093035.7e7226f2@annuminas.surriel.com> <20140430134826.GH4357@dhcp22.suse.cz> <20140430104114.4bdc588e@cuia.bos.redhat.com> <20140430120001.b4b95061ac7252a976b8a179@linux-foundation.org> <53614F3C.8020009@redhat.com> <20140430123526.bc6a229c1ea4addad1fb483d@linux-foundation.org> <20140430160218.442863e0@cuia.bos.redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Rik van Riel Cc: Michal Hocko , Masayoshi Mizuma , linux-kernel@vger.kernel.org, linux-mm@kvack.org, sandeen@redhat.com, jweiner@redhat.com, kosaki.motohiro@jp.fujitsu.com, fengguang.wu@intel.com, mpatlasov@parallels.com, Motohiro.Kosaki@us.fujitsu.com On Wed, 30 Apr 2014 16:02:18 -0400 Rik van Riel wrote: > I believe this should do the trick. > > ---8<--- > > Subject: mm,writeback: fix divide by zero in pos_ratio_polynom > > It is possible for "limit - setpoint + 1" to equal zero, leading to a > divide by zero error. Blindly adding 1 to "limit - setpoint" is not > working, so we need to actually test the divisor before calling div64. > Changelog is a bit stale. > --- > mm/page-writeback.c | 19 ++++++++++++++----- > 1 file changed, 14 insertions(+), 5 deletions(-) > > diff --git a/mm/page-writeback.c b/mm/page-writeback.c > index ef41349..37f56bb 100644 > --- a/mm/page-writeback.c > +++ b/mm/page-writeback.c > @@ -593,15 +593,20 @@ unsigned long bdi_dirty_limit(struct backing_dev_info *bdi, unsigned long dirty) > * (5) the closer to setpoint, the smaller |df/dx| (and the reverse) > * => fast response on large errors; small oscillation near setpoint > */ > -static inline long long pos_ratio_polynom(unsigned long setpoint, > +static long long pos_ratio_polynom(unsigned long setpoint, > unsigned long dirty, > unsigned long limit) > { > + unsigned long divisor; > long long pos_ratio; > long x; > > - x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT, > - limit - setpoint + 1); > + divisor = limit - setpoint; > + if (!divisor) > + divisor = 1; /* Avoid div-by-zero */ This was a consequence of 64->32 truncation and it can't happen any more, can it? -- 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