From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx173.postini.com [74.125.245.173]) by kanga.kvack.org (Postfix) with SMTP id 87D086B004D for ; Wed, 7 Nov 2012 17:46:43 -0500 (EST) Received: by mail-ea0-f169.google.com with SMTP id k11so1002305eaa.14 for ; Wed, 07 Nov 2012 14:46:42 -0800 (PST) Date: Wed, 7 Nov 2012 23:46:40 +0100 From: Michal Hocko Subject: Re: [PATCH v2] memcg: oom: fix totalpages calculation for memory.swappiness==0 Message-ID: <20121107224640.GE26382@dhcp22.suse.cz> References: <20121011085038.GA29295@dhcp22.suse.cz> <1349945859-1350-1-git-send-email-mhocko@suse.cz> <20121015220354.GA11682@dhcp22.suse.cz> <20121107141025.2ac62206.akpm@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20121107141025.2ac62206.akpm@linux-foundation.org> Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton Cc: linux-mm@kvack.org, David Rientjes , KOSAKI Motohiro , KAMEZAWA Hiroyuki , Johannes Weiner , LKML On Wed 07-11-12 14:10:25, Andrew Morton wrote: > On Tue, 16 Oct 2012 00:04:08 +0200 > Michal Hocko wrote: > > > As Kosaki correctly pointed out, the glogal reclaim doesn't have this > > issue because we _do_ swap on swappinnes==0 so the swap space has > > to be considered. So the v2 is just acks + changelog fix. > > > > Changes since v1 > > - drop a note about global swappiness affected as well from the > > changelog > > - stable needs 3.2+ rather than 3.5+ because the fe35004f has been > > backported to stable > > --- > > >From c2ae4849f09dbfda6b61472c6dd1fd8c2fe8ac81 Mon Sep 17 00:00:00 2001 > > From: Michal Hocko > > Date: Wed, 10 Oct 2012 15:46:54 +0200 > > Subject: [PATCH] memcg: oom: fix totalpages calculation for > > memory.swappiness==0 > > > > oom_badness takes totalpages argument which says how many pages are > > available and it uses it as a base for the score calculation. The value > > is calculated by mem_cgroup_get_limit which considers both limit and > > total_swap_pages (resp. memsw portion of it). > > > > This is usually correct but since fe35004f (mm: avoid swapping out > > with swappiness==0) we do not swap when swappiness is 0 which means > > that we cannot really use up all the totalpages pages. This in turn > > confuses oom score calculation if the memcg limit is much smaller than > > the available swap because the used memory (capped by the limit) is > > negligible comparing to totalpages so the resulting score is too small > > if adj!=0 (typically task with CAP_SYS_ADMIN or non zero oom_score_adj). > > A wrong process might be selected as result. > > > > The problem can be worked around by checking mem_cgroup_swappiness==0 > > and not considering swap at all in such a case. > > > > Signed-off-by: Michal Hocko > > Acked-by: David Rientjes > > Acked-by: Johannes Weiner > > Acked-by: KOSAKI Motohiro > > Acked-by: KAMEZAWA Hiroyuki > > Cc: stable [3.2+] > > That's "Cc: ", please. Will do next time. > It's unobvious from the changelog that a -stable backport is really > needed. The bug looks pretty obscure and has been there for a long > time. Yes but it is not _that_ long since fe35004f made it into stable trees (e.g. 3.2.29). The reason why we probably do not see many reports is because people didn't get used to swappiness==0 really works these days - especially with memcg where it means _really_ no swapping. > Realistically, is anyone likely to hurt from this? The primary motivation for the fix was a real report by a customer. -- Michal Hocko SUSE Labs -- 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