From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ee0-f47.google.com (mail-ee0-f47.google.com [74.125.83.47]) by kanga.kvack.org (Postfix) with ESMTP id B031F6B0035 for ; Tue, 29 Apr 2014 18:48:21 -0400 (EDT) Received: by mail-ee0-f47.google.com with SMTP id b15so768818eek.20 for ; Tue, 29 Apr 2014 15:48:20 -0700 (PDT) Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTP id r9si28488161eew.348.2014.04.29.15.48.18 for ; Tue, 29 Apr 2014 15:48:19 -0700 (PDT) Message-ID: <53602C2B.50604@redhat.com> Date: Tue, 29 Apr 2014 18:48:11 -0400 From: Rik van Riel MIME-Version: 1.0 Subject: Re: [PATCH] mm,writeback: fix divide by zero in pos_ratio_polynom References: <20140429151910.53f740ef@annuminas.surriel.com> <20140429153936.49a2710c0c2bba4d233032f2@linux-foundation.org> In-Reply-To: <20140429153936.49a2710c0c2bba4d233032f2@linux-foundation.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, sandeen@redhat.com, jweiner@redhat.com, kosaki.motohiro@jp.fujitsu.com, mhocko@suse.cz, fengguang.wu@intel.com, mpatlasov@parallels.com On 04/29/2014 06:39 PM, Andrew Morton wrote: > On Tue, 29 Apr 2014 15:19:10 -0400 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. >> >> ... >> >> --- 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; > > I'm thinking this would be better as a ulong so I don't have to worry > my pretty head over truncation things? I looked at div_*64, and the second argument is a 32 bit variable. I guess a long would be ok, since if we are dividing by more than 4 billion we don't really care :) static inline s64 div_s64(s64 dividend, s32 divisor) > --- a/mm/page-writeback.c~mm-page-writebackc-fix-divide-by-zero-in-pos_ratio_polynom-fix > +++ a/mm/page-writeback.c > @@ -597,13 +597,13 @@ static inline long long pos_ratio_polyno > unsigned long dirty, > unsigned long limit) > { > - unsigned int divisor; > + unsigned long divisor; > long long pos_ratio; > long x; > > divisor = limit - setpoint; > if (!divisor) > - divisor = 1; > + divisor = 1; /* Avoid div-by-zero */ Works for me :) -- All rights reversed -- 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