Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758065AbaD3IEU (ORCPT ); Wed, 30 Apr 2014 04:04:20 -0400 Received: from relay.parallels.com ([195.214.232.42]:43148 "EHLO relay.parallels.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751426AbaD3IEQ (ORCPT ); Wed, 30 Apr 2014 04:04:16 -0400 Message-ID: <5360AE74.7050100@parallels.com> Date: Wed, 30 Apr 2014 12:04:04 +0400 From: Maxim Patlasov User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.4.0 MIME-Version: 1.0 To: Rik van Riel , CC: , , , , , , Subject: Re: [PATCH] mm,writeback: fix divide by zero in pos_ratio_polynom References: <20140429151910.53f740ef@annuminas.surriel.com> In-Reply-To: <20140429151910.53f740ef@annuminas.surriel.com> Content-Type: text/plain; charset="ISO-8859-1"; format=flowed Content-Transfer-Encoding: 7bit X-Originating-IP: [10.30.30.94] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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(): > 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; -- 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/