Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758895AbaD3Noe (ORCPT ); Wed, 30 Apr 2014 09:44:34 -0400 Received: from mx1.redhat.com ([209.132.183.28]:56255 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758576AbaD3Nod (ORCPT ); Wed, 30 Apr 2014 09:44:33 -0400 Date: Wed, 30 Apr 2014 09:30:35 -0400 From: Rik van Riel To: Masayoshi Mizuma Cc: , , , , , , , , , Subject: [PATCH v2] mm,writeback: fix divide by zero in pos_ratio_polynom Message-ID: <20140430093035.7e7226f2@annuminas.surriel.com> In-Reply-To: <5360C9E7.6010701@jp.fujitsu.com> References: <20140429151910.53f740ef@annuminas.surriel.com> <5360C9E7.6010701@jp.fujitsu.com> Organization: Red Hat, Inc. MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 30 Apr 2014 19:01:11 +0900 Masayoshi Mizuma wrote: > Hi Rik, > > I applied your patch to linux-next kernel, then divide error happened > when I ran ltp stress test. > The divide error occurred on the following div_u64(), so the following > should be also fixed... > > static unsigned long bdi_position_ratio(struct backing_dev_info *bdi, Good catch. This patch fixes both places, and also has Andrew's improvements in both places. Andrew, this can drop in -mm instead of my previous patch and your two cleanups to it. ---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. 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 */ + 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; + 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 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/