Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1161091Ab2KNKK7 (ORCPT ); Wed, 14 Nov 2012 05:10:59 -0500 Received: from cantor2.suse.de ([195.135.220.15]:39124 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1161008Ab2KNKK5 (ORCPT ); Wed, 14 Nov 2012 05:10:57 -0500 Date: Wed, 14 Nov 2012 11:10:52 +0100 From: Michal Hocko To: Kamezawa Hiroyuki Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Johannes Weiner , Ying Han , Tejun Heo , Glauber Costa Subject: Re: [RFC 2/5] memcg: rework mem_cgroup_iter to use cgroup iterators Message-ID: <20121114101052.GD17111@dhcp22.suse.cz> References: <1352820639-13521-1-git-send-email-mhocko@suse.cz> <1352820639-13521-3-git-send-email-mhocko@suse.cz> <50A2E3B3.6080007@jp.fujitsu.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <50A2E3B3.6080007@jp.fujitsu.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: 2967 Lines: 90 On Wed 14-11-12 09:20:03, KAMEZAWA Hiroyuki wrote: > (2012/11/14 0:30), Michal Hocko wrote: [...] > > @@ -1096,30 +1096,64 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root, > > mz = mem_cgroup_zoneinfo(root, nid, zid); > > iter = &mz->reclaim_iter[reclaim->priority]; > > spin_lock(&iter->iter_lock); > > + last_visited = iter->last_visited; > > if (prev && reclaim->generation != iter->generation) { > > + if (last_visited) { > > + mem_cgroup_put(last_visited); > > + iter->last_visited = NULL; > > + } > > spin_unlock(&iter->iter_lock); > > return NULL; > > } > > - id = iter->position; > > } > > > > rcu_read_lock(); > > - css = css_get_next(&mem_cgroup_subsys, id + 1, &root->css, &id); > > - if (css) { > > - if (css == &root->css || css_tryget(css)) > > - memcg = mem_cgroup_from_css(css); > > - } else > > - id = 0; > > - rcu_read_unlock(); > > + /* > > + * Root is not visited by cgroup iterators so it needs a special > > + * treatment. > > + */ > > + if (!last_visited) { > > + css = &root->css; > > + } else { > > + struct cgroup *next_cgroup; > > + > > + next_cgroup = cgroup_next_descendant_pre( > > + last_visited->css.cgroup, > > + root->css.cgroup); > > Maybe I miss something but.... last_visited is holded by memcg's refcnt. > The cgroup pointed by css.cgroup is by cgroup's refcnt which can be freed > before memcg is freed and last_visited->css.cgroup is out of RCU cycle. > Is this safe ? Good spotted. You are right. What I need to do is to check that the last_visited is alive and restart from the root if not. Something like the bellow (incremental patch on top of this one) should help, right? diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 30efd7e..c0a91a3 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -1105,6 +1105,16 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root, spin_unlock(&iter->iter_lock); return NULL; } + /* + * memcg is still valid because we hold a reference but + * its cgroup might have vanished in the meantime so + * we have to double check it is alive and restart the + * tree walk otherwise. + */ + if (last_visited && !css_tryget(&last_visited->css)) { + mem_cgroup_put(last_visited); + last_visited = NULL; + } } rcu_read_lock(); @@ -1136,8 +1146,10 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root, if (reclaim) { struct mem_cgroup *curr = memcg; - if (last_visited) + if (last_visited) { + css_put(&last_visited->css); mem_cgroup_put(last_visited); + } if (css && !memcg) curr = mem_cgroup_from_css(css); -- Michal Hocko SUSE Labs -- 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/