From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pd0-f177.google.com (mail-pd0-f177.google.com [209.85.192.177]) by kanga.kvack.org (Postfix) with ESMTP id 308826B0035 for ; Wed, 30 Apr 2014 15:00:09 -0400 (EDT) Received: by mail-pd0-f177.google.com with SMTP id v10so2075422pde.22 for ; Wed, 30 Apr 2014 12:00:08 -0700 (PDT) Received: from mail.linuxfoundation.org (mail.linuxfoundation.org. [140.211.169.12]) by mx.google.com with ESMTP id ho7si17896598pad.233.2014.04.30.12.00.04 for ; Wed, 30 Apr 2014 12:00:04 -0700 (PDT) Date: Wed, 30 Apr 2014 12:00:01 -0700 From: Andrew Morton Subject: Re: [PATCH v3] mm,writeback: fix divide by zero in pos_ratio_polynom Message-Id: <20140430120001.b4b95061ac7252a976b8a179@linux-foundation.org> In-Reply-To: <20140430104114.4bdc588e@cuia.bos.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> 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 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. Writing a new suite of div functions sounds overkillish. At some loss of precision could we do something like if (divisor > 2^32) { divisor >>= log2(divisor) - 32; dividend >>= log2(divisor) - 32; } x = div(dividend, divisor); ? And let's uninline the sorry thing while we're in there ;) -- 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