From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ee0-f50.google.com (mail-ee0-f50.google.com [74.125.83.50]) by kanga.kvack.org (Postfix) with ESMTP id B8CCF6B0035 for ; Wed, 30 Apr 2014 10:49:08 -0400 (EDT) Received: by mail-ee0-f50.google.com with SMTP id c13so1493744eek.9 for ; Wed, 30 Apr 2014 07:49:07 -0700 (PDT) Received: from mx2.suse.de (cantor2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id 49si31170678een.5.2014.04.30.07.49.06 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Wed, 30 Apr 2014 07:49:06 -0700 (PDT) Date: Wed, 30 Apr 2014 16:49:03 +0200 From: Michal Hocko Subject: Re: [PATCH v2] mm,writeback: fix divide by zero in pos_ratio_polynom Message-ID: <20140430144903.GI4357@dhcp22.suse.cz> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <53610941.8030309@redhat.com> Sender: owner-linux-mm@kvack.org List-ID: To: Rik van Riel 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 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? -- Michal Hocko SUSE Labs -- 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