From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-we0-f176.google.com (mail-we0-f176.google.com [74.125.82.176]) by kanga.kvack.org (Postfix) with ESMTP id ADDE06B0035 for ; Wed, 30 Apr 2014 15:32:39 -0400 (EDT) Received: by mail-we0-f176.google.com with SMTP id x48so2108013wes.7 for ; Wed, 30 Apr 2014 12:32:38 -0700 (PDT) Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTP id ep15si1435629wid.87.2014.04.30.12.32.37 for ; Wed, 30 Apr 2014 12:32:38 -0700 (PDT) Message-ID: <53614F3C.8020009@redhat.com> Date: Wed, 30 Apr 2014 15:30:04 -0400 From: Rik van Riel MIME-Version: 1.0 Subject: Re: [PATCH v3] mm,writeback: fix divide by zero in pos_ratio_polynom References: <20140429151910.53f740ef@annuminas.surriel.com> <5360C9E7.6010701@jp.fujitsu.com> <20140430093035.7e7226f2@annuminas.surriel.com> <20140430134826.GH4357@dhcp22.suse.cz> <20140430104114.4bdc588e@cuia.bos.redhat.com> <20140430120001.b4b95061ac7252a976b8a179@linux-foundation.org> In-Reply-To: <20140430120001.b4b95061ac7252a976b8a179@linux-foundation.org> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton Cc: Michal Hocko , Masayoshi Mizuma , linux-kernel@vger.kernel.org, linux-mm@kvack.org, sandeen@redhat.com, jweiner@redhat.com, kosaki.motohiro@jp.fujitsu.com, fengguang.wu@intel.com, mpatlasov@parallels.com, Motohiro.Kosaki@us.fujitsu.com On 04/30/2014 03:00 PM, Andrew Morton wrote: > On Wed, 30 Apr 2014 10:41:14 -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 >> @@ -598,10 +598,15 @@ static inline long long pos_ratio_polynom(unsigned long setpoint, >> unsigned long limit) >> { >> long long pos_ratio; >> + long divisor; >> long x; >> >> + divisor = limit - setpoint; >> + if (!(s32)divisor) >> + divisor = 1; /* Avoid div-by-zero */ >> + >> x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT, >> - limit - setpoint + 1); >> + (s32)divisor); > > Doesn't this just paper over the bug one time in four billion? The > other 3999999999 times, pos_ratio_polynom() returns an incorect result? > > If it is indeed the case that pos_ratio_polynom() callers are > legitimately passing a setpoint which is more than 2^32 less than limit > then it would be better to handle that input correctly. The easy way would be by calling div64_s64 and div64_u64, which are 64 bit all the way through. Any objections? The inlined bits seem to be stubs calling the _rem variants of the functions, and discarding the remainder. -- 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