From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ee0-f41.google.com (mail-ee0-f41.google.com [74.125.83.41]) by kanga.kvack.org (Postfix) with ESMTP id 0A2BB6B003A for ; Wed, 30 Apr 2014 10:29:07 -0400 (EDT) Received: by mail-ee0-f41.google.com with SMTP id t10so1478648eei.28 for ; Wed, 30 Apr 2014 07:29:07 -0700 (PDT) Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTP id w48si31066786eel.176.2014.04.30.07.29.05 for ; Wed, 30 Apr 2014 07:29:06 -0700 (PDT) Message-ID: <5361081A.5040602@redhat.com> Date: Wed, 30 Apr 2014 10:26:34 -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> In-Reply-To: <20140430134826.GH4357@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 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? Good point. I guess we need these divisors to be "int" and "unsigned int" respectively, since those are the arguments given to div_s64 and div_u64. Let me send a v3 right now. >> x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT, >> - limit - setpoint + 1); >> + divisor); >> pos_ratio = x; >> pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT; >> pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT; >> @@ -842,8 +847,12 @@ static unsigned long bdi_position_ratio(struct backing_dev_info *bdi, >> x_intercept = bdi_setpoint + span; >> >> if (bdi_dirty < x_intercept - span / 4) { >> + unsigned long divisor = x_intercept - bdi_setpoint; > > Same here. > >> + if (!divisor) >> + divisor = 1; /* Avoid div-by-zero */ >> + >> pos_ratio = div_u64(pos_ratio * (x_intercept - bdi_dirty), >> - x_intercept - bdi_setpoint + 1); >> + divisor); >> } else >> pos_ratio /= 4; >> > -- 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