From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-we0-f172.google.com (mail-we0-f172.google.com [74.125.82.172]) by kanga.kvack.org (Postfix) with ESMTP id 3B6E86B0038 for ; Wed, 30 Apr 2014 11:31:13 -0400 (EDT) Received: by mail-we0-f172.google.com with SMTP id u57so1876752wes.17 for ; Wed, 30 Apr 2014 08:31:12 -0700 (PDT) Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTP id k2si584274wiz.12.2014.04.30.08.31.10 for ; Wed, 30 Apr 2014 08:31:11 -0700 (PDT) Message-ID: <53610E18.8050809@redhat.com> Date: Wed, 30 Apr 2014 10:52:08 -0400 From: Rik van Riel MIME-Version: 1.0 Subject: Re: [PATCH v2] 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> <53610941.8030309@redhat.com> <20140430144903.GI4357@dhcp22.suse.cz> In-Reply-To: <20140430144903.GI4357@dhcp22.suse.cz> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: Masayoshi Mizuma , linux-kernel@vger.kernel.org, linux-mm@kvack.org, sandeen@redhat.com, akpm@linux-foundation.org, jweiner@redhat.com, kosaki.motohiro@jp.fujitsu.com, fengguang.wu@intel.com, mpatlasov@parallels.com, Motohiro.Kosaki@us.fujitsu.com On 04/30/2014 10:49 AM, Michal Hocko wrote: > On Wed 30-04-14 10:31:29, Rik van Riel wrote: >> On 04/30/2014 09:48 AM, Michal Hocko wrote: >>> On Wed 30-04-14 09:30:35, Rik van Riel wrote: >>> [...] >>>> 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. >>>> >>>> Signed-off-by: Rik van Riel >>>> --- >>>> mm/page-writeback.c | 13 +++++++++++-- >>>> 1 file changed, 11 insertions(+), 2 deletions(-) >>>> >>>> diff --git a/mm/page-writeback.c b/mm/page-writeback.c >>>> index ef41349..f98a297 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 long divisor; >>>> long long pos_ratio; >>>> long x; >>>> >>>> + divisor = limit - setpoint; >>>> + if (!divisor) >>>> + divisor = 1; /* Avoid div-by-zero */ >>>> + >>> >>> This is still prone to u64 -> s32 issue, isn't it? >>> What was the original problem anyway? Was it really setpoint > limit or >>> rather the overflow? >> >> Thinking about it some more, is it possible that >> limit and/or setpoint are larger than 32 bits, but >> the difference between them is not? >> >> In that case, truncating both to 32 bits before >> doing the subtraction would be troublesome, and >> it would be better to do a cast in the comparison: >> >> if (!(s32)divisor) >> divisor = 1; > > How is that any different than defining divisor as 32b directly? For unsigned, it probably doesn't make a difference. For signed int vs unsigned long, I wonder if there is a corner case where casting the second to the first before doing the "limit - setpoint" calculation can lead to a different outcome... -- 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