From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ee0-f47.google.com (mail-ee0-f47.google.com [74.125.83.47]) by kanga.kvack.org (Postfix) with ESMTP id 20FE06B0035 for ; Thu, 30 Jan 2014 12:29:14 -0500 (EST) Received: by mail-ee0-f47.google.com with SMTP id d49so1758778eek.34 for ; Thu, 30 Jan 2014 09:29:13 -0800 (PST) Received: from zene.cmpxchg.org (zene.cmpxchg.org. [2a01:238:4224:fa00:ca1f:9ef3:caee:a2bd]) by mx.google.com with ESMTPS id h45si12184506eeo.235.2014.01.30.09.29.12 for (version=TLSv1 cipher=RC4-SHA bits=128/128); Thu, 30 Jan 2014 09:29:12 -0800 (PST) Date: Thu, 30 Jan 2014 12:29:06 -0500 From: Johannes Weiner Subject: Re: [RFC 4/5] memcg: make sure that memcg is not offline when charging Message-ID: <20140130172906.GE6963@cmpxchg.org> References: <1387295130-19771-1-git-send-email-mhocko@suse.cz> <1387295130-19771-5-git-send-email-mhocko@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1387295130-19771-5-git-send-email-mhocko@suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: Andrew Morton , KAMEZAWA Hiroyuki , KOSAKI Motohiro , LKML , linux-mm@kvack.org On Tue, Dec 17, 2013 at 04:45:29PM +0100, Michal Hocko wrote: > The current charge path might race with memcg offlining because holding > css reference doesn't stop css offline. As a result res counter might be > charged after mem_cgroup_reparent_charges (called from memcg css_offline > callback) and so the charge would never be freed. This has been worked > around by 96f1c58d8534 (mm: memcg: fix race condition between memcg > teardown and swapin) which tries to catch such a leaked charges later > during css_free. It is more optimal to heal this race in the long term > though. We already deal with the race, so IMO the only outstanding improvement is to take advantage of the teardown synchronization provided by the cgroup core and get rid of our one-liner workaround in .css_free. > In order to make this raceless we would need to hold rcu_read_lock since > css_tryget until res_counter_charge. This is not so easy unfortunately > because mem_cgroup_do_charge might sleep so we would need to do drop rcu > lock and do css_tryget tricks after each reclaim. Yes, why not? > This patch addresses the issue by introducing memcg->offline flag > which is set from mem_cgroup_css_offline callback before the pages are > reparented. mem_cgroup_do_charge checks the flag before res_counter > is charged inside rcu read section. mem_cgroup_css_offline uses > synchronize_rcu to let all preceding chargers finish while all the new > ones will see the group offline already and back out. > > Callers are then updated to retry with a new memcg which is fallback to > mem_cgroup_from_task(current). > > The only exception is mem_cgroup_do_precharge which should never see > this race because it is called from cgroup {can_}attach callbacks and so > the whole cgroup cannot go away. > > Signed-off-by: Michal Hocko > --- > mm/memcontrol.c | 58 ++++++++++++++++++++++++++++++++++++++++++++++++++++++--- > 1 file changed, 55 insertions(+), 3 deletions(-) That makes no sense to me. It's a lateral move in functionality and cgroup integration, but more complicated. -- 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