From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753382AbaANBw7 (ORCPT ); Mon, 13 Jan 2014 20:52:59 -0500 Received: from mail-pa0-f43.google.com ([209.85.220.43]:63801 "EHLO mail-pa0-f43.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752263AbaANBw4 (ORCPT ); Mon, 13 Jan 2014 20:52:56 -0500 Date: Mon, 13 Jan 2014 17:52:30 -0800 (PST) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Michal Hocko cc: Johannes Weiner , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: [PATCH 2/3] mm/memcg: fix endless iteration in reclaim In-Reply-To: Message-ID: References: User-Agent: Alpine 2.11 (LSU 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On one home machine I can easily reproduce (by rmdir of memcgdir during reclaim) multiple processes stuck looping forever in mem_cgroup_iter(): __mem_cgroup_iter_next() keeps selecting the memcg being destroyed, fails to tryget it, returns NULL to mem_cgroup_iter(), which goes around again. It's better to err on the side of leaving the loop too soon than never when such races occur: once we've served prev (using root if none), get out the next time __mem_cgroup_iter_next() cannot deliver. Signed-off-by: Hugh Dickins --- Securing the tree iterator against such races is difficult, I've certainly got it wrong myself before. Although the bug is real, and deserves a Cc stable, you may want to play around with other solutions before committing to this one. The current iterator goes back to v3.12: I'm really not sure if v3.11 was good or not - I never saw the problem in the vanilla kernel, but with Google mods in we also had to make an adjustment, there to stop __mem_cgroup_iter() being called endlessly from the reclaim level. mm/memcontrol.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) --- mmotm/mm/memcontrol.c 2014-01-10 18:25:02.236448954 -0800 +++ linux/mm/memcontrol.c 2014-01-12 22:21:10.700570471 -0800 @@ -1254,8 +1252,11 @@ struct mem_cgroup *mem_cgroup_iter(struc reclaim->generation = iter->generation; } - if (prev && !memcg) + if (!memcg) { + if (!prev) + memcg = root; goto out_unlock; + } } out_unlock: rcu_read_unlock();