Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754822Ab3EQGdy (ORCPT ); Fri, 17 May 2013 02:33:54 -0400 Received: from mx1.redhat.com ([209.132.183.28]:18324 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752834Ab3EQGdx (ORCPT ); Fri, 17 May 2013 02:33:53 -0400 Date: Fri, 17 May 2013 16:33:40 +1000 From: Dave Chinner To: Oskar Andero Cc: "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , "Lekanovic, Radovan" , David Rientjes , Glauber Costa , Andrew Morton , Hugh Dickins , Greg Kroah-Hartman Subject: Re: [PATCH] mm: vmscan: handle any negative return value from scan_objects Message-ID: <20130517063340.GE11167@devil.localdomain> References: <1368693736-15486-1-git-send-email-oskar.andero@sonymobile.com> <20130516115212.GC11167@devil.localdomain> <20130516122752.GG24072@caracas.corpusers.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130516122752.GG24072@caracas.corpusers.net> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2358 Lines: 60 On Thu, May 16, 2013 at 02:27:52PM +0200, Oskar Andero wrote: > On 13:52 Thu 16 May , Dave Chinner wrote: > > On Thu, May 16, 2013 at 10:42:16AM +0200, Oskar Andero wrote: > > > The shrinkers must return -1 to indicate that it is busy. Instead, treat > > > any negative value as busy. > > > > Why? The API defines return condition for aborting a scan and gives > > a specific value for doing that. i.e. explain why should change the > > API to over-specify the 'abort scan" return value like this. > > As I pointed out earlier, looking in to the code (from master): > if (shrink_ret == -1) > break; > if (shrink_ret < nr_before) > ret += nr_before - shrink_ret; > > This piece of code lacks a sanity check and will only function if shrink_ret > is either greater than zero or exactly -1. If shrink_ret is e.g. -2 this will > lead to undefined behaviour. If a shrinker is returning -2 then the shrinker is broken and needs fixing. > > FWIW, using "any" negative number for "abort scan" is a bad API > > design decision. It means that in future we can't introduce > > different negative return values in the API if we have a new to. > > i.e. each specific negative return value needs to have the potential > > for defining a different behaviour. > > An alternative to my patch would be to add: > if (shrink_ret < -1) > /* handle illegal return code in some way */ How? We have one valid negative return code. WTF are we supposed to do if a shrinker is passing undefined return values? IOWs, the only sane thing to do is: BUG_ON(shrink_ret < -1); > > So if any change needs to be made, it is to change the -1 return > > value to an enum and have the shrinkers return that enum when they > > want an abort. > > I am all for an enum, but I still believe we should handle the case where > the shrinkers return something wicked. Which bit of "broken shrinkers need to be fixed" don't you understand? A BUG_ON() will make sure they get fixed - anything else that allows broken shrinkers to continue functioning is a completely unacceptible solution. -Dave. -- Dave Chinner dchinner@redhat.com -- 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/