Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757910AbaD2Wjk (ORCPT ); Tue, 29 Apr 2014 18:39:40 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:35468 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756224AbaD2Wjj (ORCPT ); Tue, 29 Apr 2014 18:39:39 -0400 Date: Tue, 29 Apr 2014 15:39:36 -0700 From: Andrew Morton To: Rik van Riel Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, sandeen@redhat.com, jweiner@redhat.com, kosaki.motohiro@jp.fujitsu.com, mhocko@suse.cz, fengguang.wu@intel.com, mpatlasov@parallels.com Subject: Re: [PATCH] mm,writeback: fix divide by zero in pos_ratio_polynom Message-Id: <20140429153936.49a2710c0c2bba4d233032f2@linux-foundation.org> In-Reply-To: <20140429151910.53f740ef@annuminas.surriel.com> References: <20140429151910.53f740ef@annuminas.surriel.com> X-Mailer: Sylpheed 3.2.0beta5 (GTK+ 2.24.10; x86_64-pc-linux-gnu) 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 Tue, 29 Apr 2014 15:19:10 -0400 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. > > ... > > --- 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; I'm thinking this would be better as a ulong so I don't have to worry my pretty head over truncation things? > 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; --- a/mm/page-writeback.c~mm-page-writebackc-fix-divide-by-zero-in-pos_ratio_polynom-fix +++ a/mm/page-writeback.c @@ -597,13 +597,13 @@ static inline long long pos_ratio_polyno unsigned long dirty, unsigned long limit) { - unsigned int divisor; + unsigned long divisor; long long pos_ratio; long x; divisor = limit - setpoint; if (!divisor) - divisor = 1; + divisor = 1; /* Avoid div-by-zero */ x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT, divisor); _ -- 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/