From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ie0-f172.google.com (mail-ie0-f172.google.com [209.85.223.172]) by kanga.kvack.org (Postfix) with ESMTP id 47EE06B0037 for ; Wed, 30 Apr 2014 15:35:30 -0400 (EDT) Received: by mail-ie0-f172.google.com with SMTP id as1so2469899iec.31 for ; Wed, 30 Apr 2014 12:35:29 -0700 (PDT) Received: from mail.linuxfoundation.org (mail.linuxfoundation.org. [140.211.169.12]) by mx.google.com with ESMTP id u6si19932658icp.38.2014.04.30.12.35.28 for ; Wed, 30 Apr 2014 12:35:28 -0700 (PDT) Date: Wed, 30 Apr 2014 12:35:26 -0700 From: Andrew Morton Subject: Re: [PATCH v3] mm,writeback: fix divide by zero in pos_ratio_polynom Message-Id: <20140430123526.bc6a229c1ea4addad1fb483d@linux-foundation.org> In-Reply-To: <53614F3C.8020009@redhat.com> 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> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Rik van Riel 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 On Wed, 30 Apr 2014 15:30:04 -0400 Rik van Riel wrote: > On 04/30/2014 03:00 PM, Andrew Morton wrote: > > On Wed, 30 Apr 2014 10:41:14 -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 > >> @@ -598,10 +598,15 @@ static inline long long pos_ratio_polynom(unsigned long setpoint, > >> unsigned long limit) > >> { > >> long long pos_ratio; > >> + long divisor; > >> long x; > >> > >> + divisor = limit - setpoint; > >> + if (!(s32)divisor) > >> + divisor = 1; /* Avoid div-by-zero */ > >> + > >> x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT, > >> - limit - setpoint + 1); > >> + (s32)divisor); > > > > Doesn't this just paper over the bug one time in four billion? The > > other 3999999999 times, pos_ratio_polynom() returns an incorect result? > > > > If it is indeed the case that pos_ratio_polynom() callers are > > legitimately passing a setpoint which is more than 2^32 less than limit > > then it would be better to handle that input correctly. > > 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... -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org