Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756649Ab3EPIUS (ORCPT ); Thu, 16 May 2013 04:20:18 -0400 Received: from seldrel01.sonyericsson.com ([212.209.106.2]:11249 "EHLO seldrel01.sonyericsson.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755857Ab3EPIUF (ORCPT ); Thu, 16 May 2013 04:20:05 -0400 From: Oskar Andero Date: Thu, 16 May 2013 10:20:00 +0200 To: Glauber Costa CC: "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , Hugh Dickins , Greg Kroah-Hartman , "Lekanovic, Radovan" , David Rientjes , Dave Chinner , Mel Gorman Subject: Re: [RFC PATCH 0/2] return value from shrinkers Message-ID: <20130516082000.GE24072@caracas.corpusers.net> References: <1368454595-5121-1-git-send-email-oskar.andero@sonymobile.com> <5192523B.7030805@parallels.com> <20130515141057.GA24072@caracas.corpusers.net> <51939948.3040307@parallels.com> <20130515144704.GC24072@caracas.corpusers.net> <5193A085.3020309@parallels.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <5193A085.3020309@parallels.com> 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: 4963 Lines: 109 On 16:49 Wed 15 May , Glauber Costa wrote: > On 05/15/2013 06:47 PM, Oskar Andero wrote: > > On 16:18 Wed 15 May , Glauber Costa wrote: > >> On 05/15/2013 06:10 PM, Oskar Andero wrote: > >>> On 17:03 Tue 14 May , Glauber Costa wrote: > >>>> On 05/13/2013 06:16 PM, Oskar Andero wrote: > >>>>> Hi, > >>>>> > >>>>> In a previous discussion on lkml it was noted that the shrinkers use the > >>>>> magic value "-1" to signal that something went wrong. > >>>>> > >>>>> This patch-set implements the suggestion of instead using errno.h values > >>>>> to return something more meaningful. > >>>>> > >>>>> The first patch simply changes the check from -1 to any negative value and > >>>>> updates the comment accordingly. > >>>>> > >>>>> The second patch updates the shrinkers to return an errno.h value instead > >>>>> of -1. Since this one spans over many different areas I need input on what is > >>>>> a meaningful return value. Right now I used -EBUSY on everything for consitency. > >>>>> > >>>>> What do you say? Is this a good idea or does it make no sense at all? > >>>>> > >>>>> Thanks! > >>>>> > >>>> > >>>> Right now me and Dave are completely reworking the way shrinkers > >>>> operate. I suggest, first of all, that you take a look at that cautiously. > >>> > >>> Sounds good. Where can one find the code for that? > >>> > >> linux-mm, linux-fsdevel > >> > >> Subject is "kmemcg shrinkers", but only the second part is memcg related. > >> > >>>> On the specifics of what you are doing here, what would be the benefit > >>>> of returning something other than -1 ? Is there anything we would do > >>>> differently for a return value lesser than 1? > >>> > >>> Firstly, what bugs me is the magic and unintuitiveness of using -1 rather than a > >>> more descriptive error code. IMO, even a #define SHRINK_ERROR -1 in some header > >>> file would be better. > >>> > >>> Expanding the test to <0 will open up for more granular error checks, > >>> like -EAGAIN, -EBUSY and so on. Currently, they would all be treated the same, > >>> but maybe in the future we would like to handle them differently? > >>> > >> > >> Then in the future we change it. > >> This is not a user visible API, we are free to change it at any time, > >> under any conditions. There is only value in supporting different error > >> codes if we intend to do something different about it. Otherwise, it is > >> just churn. > >> > >> Moreover, -1 does not necessarily mean error. It means "stop shrinking". > >> There are many non-error conditions in which it could happen. > >> > > > > Sure, maybe errno.h is not the right way to go. So, why not add the #define > > instead? E.g. STOP_SHRINKING or something better than -1. > > > >>> Finally, looking at the code: > >>> if (shrink_ret == -1) > >>> break; > >>> if (shrink_ret < nr_before) > >>> ret += nr_before - shrink_ret; > >>> > >>> This piece of code will only function if shrink_ret is either greater than zero > >>> or -1. If shrink_ret is -2 this will lead to undefined behaviour. > >>> > >> Except it never is. But since we are touching this code anyway, I see no > >> problems in expanding the test. What I don't see the point for, is the > >> other patch in your series in which you return error codes. > >> > >>>> So far, shrink_slab behaves the same, you are just expanding the test. > >>>> If you really want to push this through, I would suggest coming up with > >>>> a more concrete reason for why this is wanted. > >>> > >>> I don't know how well this patch is aligned with your current rework, but > >>> based on my comments above, I don't see a reason for not taking it. > >>> > >> I see no objections for PATCH #1 that expands the check, as a cautionary > >> measure. But I will oppose returning error codes from shrinkers without > >> a solid reason for doing so (meaning a use case in which we really > >> threat one of the errors differently) > > > > Sorry for being over-zealous about the return codes and I understand > > that it is really a minor thing and possibly also a philosophical > > question. My only "solid" reasons are unintuiveness and readability. > > That is how I came across it in the first place. > > > > If no-one backs me up on this I will drop the second patch and resend > > the first patch without RFC prefix. > > > If you are willing to wait a bit until it finally gets merged, please > send it against my memcg.git in kernel.org (branch > kmemcg-lru-shrinkers). I can carry your patch in our series. Alright. I will apply PATCH 1/2 ontop of your kmemcg-lru-shrinker branch and send it to you offline. Thanks! -Oskar -- 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/