Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965100AbaD3Uph (ORCPT ); Wed, 30 Apr 2014 16:45:37 -0400 Received: from mx1.redhat.com ([209.132.183.28]:64386 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759042AbaD3Upf (ORCPT ); Wed, 30 Apr 2014 16:45:35 -0400 Date: Wed, 30 Apr 2014 16:42:55 -0400 From: Rik van Riel To: Andrew Morton Cc: Michal Hocko , Masayoshi Mizuma , linux-kernel@vger.kernel.org, linux-mm@kvack.org, sandeen@redhat.com, jweiner@redhat.com, kosaki.motohiro@jp.fujitsu.com, fengguang.wu@intel.com, mpatlasov@parallels.com, Motohiro.Kosaki@us.fujitsu.com Subject: [PATCH v5] mm,writeback: fix divide by zero in pos_ratio_polynom Message-ID: <20140430164255.7a753a8e@cuia.bos.redhat.com> In-Reply-To: <20140430131353.fa9f49604ea39425bc93c24a@linux-foundation.org> References: <20140429151910.53f740ef@annuminas.surriel.com> <5360C9E7.6010701@jp.fujitsu.com> <20140430093035.7e7226f2@annuminas.surriel.com> <20140430134826.GH4357@dhcp22.suse.cz> <20140430104114.4bdc588e@cuia.bos.redhat.com> <20140430120001.b4b95061ac7252a976b8a179@linux-foundation.org> <53614F3C.8020009@redhat.com> <20140430123526.bc6a229c1ea4addad1fb483d@linux-foundation.org> <20140430160218.442863e0@cuia.bos.redhat.com> <20140430131353.fa9f49604ea39425bc93c24a@linux-foundation.org> 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 13:13:53 -0700 Andrew Morton wrote: > This was a consequence of 64->32 truncation and it can't happen any > more, can it? Andrew, this is cleaner indeed :) Masayoshi-san, does the bug still happen with this version, or does this fix the problem? ---8<--- Subject: mm,writeback: fix divide by zero in pos_ratio_polynom It is possible for "limit - setpoint + 1" to equal zero, after getting truncated to a 32 bit variable, and resulting in a divide by zero error. Using the fully 64 bit divide functions avoids this problem. Also uninline pos_ratio_polynom, at Andrew's request. Signed-off-by: Rik van Riel --- mm/page-writeback.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/mm/page-writeback.c b/mm/page-writeback.c index ef41349..a4317da 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -593,14 +593,14 @@ unsigned long bdi_dirty_limit(struct backing_dev_info *bdi, unsigned long dirty) * (5) the closer to setpoint, the smaller |df/dx| (and the reverse) * => fast response on large errors; small oscillation near setpoint */ -static inline long long pos_ratio_polynom(unsigned long setpoint, +static long long pos_ratio_polynom(unsigned long setpoint, unsigned long dirty, unsigned long limit) { long long pos_ratio; long x; - x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT, + x = div64_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT, limit - setpoint + 1); pos_ratio = x; pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT; @@ -842,7 +842,7 @@ static unsigned long bdi_position_ratio(struct backing_dev_info *bdi, x_intercept = bdi_setpoint + span; if (bdi_dirty < x_intercept - span / 4) { - pos_ratio = div_u64(pos_ratio * (x_intercept - bdi_dirty), + pos_ratio = div64_u64(pos_ratio * (x_intercept - bdi_dirty), x_intercept - bdi_setpoint + 1); } 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/