Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759037AbaD3Nsb (ORCPT ); Wed, 30 Apr 2014 09:48:31 -0400 Received: from cantor2.suse.de ([195.135.220.15]:45599 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757983AbaD3Nsa (ORCPT ); Wed, 30 Apr 2014 09:48:30 -0400 Date: Wed, 30 Apr 2014 15:48:26 +0200 From: Michal Hocko To: Rik van Riel Cc: Masayoshi Mizuma , linux-kernel@vger.kernel.org, linux-mm@kvack.org, sandeen@redhat.com, akpm@linux-foundation.org, jweiner@redhat.com, kosaki.motohiro@jp.fujitsu.com, fengguang.wu@intel.com, mpatlasov@parallels.com, Motohiro.Kosaki@us.fujitsu.com Subject: Re: [PATCH v2] mm,writeback: fix divide by zero in pos_ratio_polynom Message-ID: <20140430134826.GH4357@dhcp22.suse.cz> References: <20140429151910.53f740ef@annuminas.surriel.com> <5360C9E7.6010701@jp.fujitsu.com> <20140430093035.7e7226f2@annuminas.surriel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140430093035.7e7226f2@annuminas.surriel.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 30-04-14 09:30:35, Rik van Riel wrote: [...] > 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. > > Signed-off-by: Rik van Riel > --- > mm/page-writeback.c | 13 +++++++++++-- > 1 file changed, 11 insertions(+), 2 deletions(-) > > diff --git a/mm/page-writeback.c b/mm/page-writeback.c > index ef41349..f98a297 100644 > --- a/mm/page-writeback.c > +++ b/mm/page-writeback.c > @@ -597,11 +597,16 @@ static inline long long pos_ratio_polynom(unsigned long setpoint, > unsigned long dirty, > unsigned long limit) > { > + unsigned long divisor; > long long pos_ratio; > long x; > > + divisor = limit - setpoint; > + if (!divisor) > + divisor = 1; /* Avoid div-by-zero */ > + This is still prone to u64 -> s32 issue, isn't it? What was the original problem anyway? Was it really setpoint > limit or rather the overflow? > x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT, > - limit - setpoint + 1); > + divisor); > pos_ratio = x; > pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT; > pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT; > @@ -842,8 +847,12 @@ static unsigned long bdi_position_ratio(struct backing_dev_info *bdi, > x_intercept = bdi_setpoint + span; > > if (bdi_dirty < x_intercept - span / 4) { > + unsigned long divisor = x_intercept - bdi_setpoint; Same here. > + if (!divisor) > + divisor = 1; /* Avoid div-by-zero */ > + > pos_ratio = div_u64(pos_ratio * (x_intercept - bdi_dirty), > - x_intercept - bdi_setpoint + 1); > + divisor); > } else > pos_ratio /= 4; > -- Michal Hocko SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/