Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758056AbaD3Ie6 (ORCPT ); Wed, 30 Apr 2014 04:34:58 -0400 Received: from relay.parallels.com ([195.214.232.42]:58788 "EHLO relay.parallels.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751818AbaD3Ie5 (ORCPT ); Wed, 30 Apr 2014 04:34:57 -0400 Message-ID: <5360B5A5.5060101@parallels.com> Date: Wed, 30 Apr 2014 12:34:45 +0400 From: Maxim Patlasov User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.4.0 MIME-Version: 1.0 To: Michal Hocko CC: Rik van Riel , , , , , , , Subject: Re: [PATCH] mm,writeback: fix divide by zero in pos_ratio_polynom References: <20140429151910.53f740ef@annuminas.surriel.com> <5360AE74.7050100@parallels.com> <20140430081256.GA4357@dhcp22.suse.cz> In-Reply-To: <20140430081256.GA4357@dhcp22.suse.cz> Content-Type: text/plain; charset="ISO-8859-1"; format=flowed Content-Transfer-Encoding: 7bit X-Originating-IP: [10.30.30.94] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/30/2014 12:12 PM, Michal Hocko wrote: > On Wed 30-04-14 12:04:04, Maxim Patlasov wrote: >> Hi Rik! >> >> On 04/29/2014 11:19 PM, 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. >> The patch looks correct, but I'm afraid it can hide an actual bug in a >> caller of pos_ratio_polynom(). The latter is not intended for setpoint > >> limit. All callers take pains to ensure that setpoint <= limit. Look, for >> example, at global_dirty_limits(): > The bug might trigger even if setpoint < limit because the result is > trucated to s32 and I guess this is what is going on here? > Is (limit - setpoint + 1) > 4G possible? Yes, you are right. Probably the problem came from s32 overflow. > >>> if (background >= dirty) >>> background = dirty / 2; >> If you ever encountered "limit - setpoint + 1" equal zero, it may be worthy >> to investigate how you came to setpoint greater than limit. >> >> Thanks, >> Maxim >> >>> Signed-off-by: Rik van Riel >>> Cc: stable@vger.kernel.org >>> --- >>> mm/page-writeback.c | 7 ++++++- >>> 1 file changed, 6 insertions(+), 1 deletion(-) >>> >>> diff --git a/mm/page-writeback.c b/mm/page-writeback.c >>> index ef41349..2682516 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 int divisor; >>> long long pos_ratio; >>> long x; >>> + divisor = limit - setpoint; >>> + if (!divisor) >>> + divisor = 1; >>> + >>> 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; -- 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/