From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753471AbaA3R3N (ORCPT ); Thu, 30 Jan 2014 12:29:13 -0500 Received: from zene.cmpxchg.org ([85.214.230.12]:54943 "EHLO zene.cmpxchg.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753330AbaA3R3L (ORCPT ); Thu, 30 Jan 2014 12:29:11 -0500 Date: Thu, 30 Jan 2014 12:29:06 -0500 From: Johannes Weiner To: Michal Hocko Cc: Andrew Morton , KAMEZAWA Hiroyuki , KOSAKI Motohiro , LKML , linux-mm@kvack.org 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: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.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.