Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759589AbaD3Uf1 (ORCPT ); Wed, 30 Apr 2014 16:35:27 -0400 Received: from mx1.redhat.com ([209.132.183.28]:57819 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759042AbaD3UfX (ORCPT ); Wed, 30 Apr 2014 16:35:23 -0400 Message-ID: <53615DEE.90808@redhat.com> Date: Wed, 30 Apr 2014 16:32:46 -0400 From: Rik van Riel User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.2.0 MIME-Version: 1.0 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 Subject: Re: [PATCH v4] 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> <53614F3C.8020009@redhat.com> <20140430123526.bc6a229c1ea4addad1fb483d@linux-foundation.org> <20140430160218.442863e0@cuia.bos.redhat.com> <20140430131353.fa9f49604ea39425bc93c24a@linux-foundation.org> In-Reply-To: <20140430131353.fa9f49604ea39425bc93c24a@linux-foundation.org> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/30/2014 04:13 PM, Andrew Morton wrote: > On Wed, 30 Apr 2014 16:02:18 -0400 Rik van Riel wrote: > >> I believe this should do the trick. >> >> ---8<--- >> >> Subject: mm,writeback: fix divide by zero in pos_ratio_polynom >> >> 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. > > Changelog is a bit stale. Will update. >> -static inline long long pos_ratio_polynom(unsigned long setpoint, >> +static long long pos_ratio_polynom(unsigned long setpoint, >> unsigned long dirty, >> unsigned long limit) >> { >> + unsigned long divisor; >> long long pos_ratio; >> long x; >> >> - x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT, >> - limit - setpoint + 1); >> + divisor = limit - setpoint; >> + if (!divisor) >> + divisor = 1; /* Avoid div-by-zero */ > > This was a consequence of 64->32 truncation and it can't happen any > more, can it? That is a good question. Looking at the code some more, I guess it may indeed be exclusively due to the truncation, and we can go back to the older code, just with the fully 64 bit divide functions... Good thing Masayoshi-san has a reproducer :) -- 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/