Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932241AbaD3INJ (ORCPT ); Wed, 30 Apr 2014 04:13:09 -0400 Received: from cantor2.suse.de ([195.135.220.15]:39182 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751769AbaD3IND (ORCPT ); Wed, 30 Apr 2014 04:13:03 -0400 Date: Wed, 30 Apr 2014 10:12:56 +0200 From: Michal Hocko To: Maxim Patlasov Cc: Rik van Riel , 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 Subject: Re: [PATCH] mm,writeback: fix divide by zero in pos_ratio_polynom Message-ID: <20140430081256.GA4357@dhcp22.suse.cz> References: <20140429151910.53f740ef@annuminas.surriel.com> <5360AE74.7050100@parallels.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5360AE74.7050100@parallels.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 12:04:04, Maxim Patlasov wrote: > Hi Rik! > > On 04/29/2014 11:19 PM, Rik van Riel wrote: > >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. > > The patch looks correct, but I'm afraid it can hide an actual bug in a > caller of pos_ratio_polynom(). The latter is not intended for setpoint > > limit. All callers take pains to ensure that setpoint <= limit. Look, for > example, at global_dirty_limits(): The bug might trigger even if setpoint < limit because the result is trucated to s32 and I guess this is what is going on here? Is (limit - setpoint + 1) > 4G possible? > > > if (background >= dirty) > > background = dirty / 2; > > If you ever encountered "limit - setpoint + 1" equal zero, it may be worthy > to investigate how you came to setpoint greater than limit. > > Thanks, > Maxim > > > > >Signed-off-by: Rik van Riel > >Cc: stable@vger.kernel.org > >--- > > mm/page-writeback.c | 7 ++++++- > > 1 file changed, 6 insertions(+), 1 deletion(-) > > > >diff --git a/mm/page-writeback.c b/mm/page-writeback.c > >index ef41349..2682516 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 int divisor; > > long long pos_ratio; > > long x; > >+ divisor = limit - setpoint; > >+ if (!divisor) > >+ divisor = 1; > >+ > > 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; > -- 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/