Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1946063AbaD3UFb (ORCPT ); Wed, 30 Apr 2014 16:05:31 -0400 Received: from mx1.redhat.com ([209.132.183.28]:43872 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759355AbaD3UFa (ORCPT ); Wed, 30 Apr 2014 16:05:30 -0400 Date: Wed, 30 Apr 2014 16:02:18 -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 v4] mm,writeback: fix divide by zero in pos_ratio_polynom Message-ID: <20140430160218.442863e0@cuia.bos.redhat.com> In-Reply-To: <20140430123526.bc6a229c1ea4addad1fb483d@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> 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 12:35:26 -0700 Andrew Morton wrote: > > The easy way would be by calling div64_s64 and div64_u64, > > which are 64 bit all the way through. > > > > Any objections? > > Sounds good to me. > > > The inlined bits seem to be stubs calling the _rem variants > > of the functions, and discarding the remainder. > > I was referring to pos_ratio_polynom(). The compiler will probably be > uninlining it anyway, but still... I believe this should do the trick. ---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 | 19 ++++++++++++++----- 1 file changed, 14 insertions(+), 5 deletions(-) diff --git a/mm/page-writeback.c b/mm/page-writeback.c index ef41349..37f56bb 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -593,15 +593,20 @@ 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) { + unsigned long divisor; long long pos_ratio; long x; - x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT, - limit - setpoint + 1); + divisor = limit - setpoint; + if (!divisor) + divisor = 1; /* Avoid div-by-zero */ + + x = div64_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT, + 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) { - pos_ratio = div_u64(pos_ratio * (x_intercept - bdi_dirty), - x_intercept - bdi_setpoint + 1); + unsigned long divisor = x_intercept - bdi_setpoint; + if (!divisor) + divisor = 1; /* Avoid div-by-zero */ + + pos_ratio = div64_u64(pos_ratio * (x_intercept - bdi_dirty), + 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/