From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-la0-f42.google.com (mail-la0-f42.google.com [209.85.215.42]) by kanga.kvack.org (Postfix) with ESMTP id 86A9B6B0035 for ; Wed, 30 Apr 2014 04:04:19 -0400 (EDT) Received: by mail-la0-f42.google.com with SMTP id mc6so976048lab.1 for ; Wed, 30 Apr 2014 01:04:18 -0700 (PDT) Received: from relay.parallels.com (relay.parallels.com. [195.214.232.42]) by mx.google.com with ESMTPS id tv3si9498732lbb.7.2014.04.30.01.04.16 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 30 Apr 2014 01:04:17 -0700 (PDT) Message-ID: <5360AE74.7050100@parallels.com> Date: Wed, 30 Apr 2014 12:04:04 +0400 From: Maxim Patlasov MIME-Version: 1.0 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 Sender: owner-linux-mm@kvack.org List-ID: To: Rik van Riel , linux-kernel@vger.kernel.org Cc: linux-mm@kvack.org, sandeen@redhat.com, akpm@linux-foundation.org, jweiner@redhat.com, kosaki.motohiro@jp.fujitsu.com, mhocko@suse.cz, fengguang.wu@intel.com 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, 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