From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-yh0-f41.google.com (mail-yh0-f41.google.com [209.85.213.41]) by kanga.kvack.org (Postfix) with ESMTP id 3AD326B0031 for ; Mon, 13 Jan 2014 20:52:58 -0500 (EST) Received: by mail-yh0-f41.google.com with SMTP id i57so1240529yha.14 for ; Mon, 13 Jan 2014 17:52:58 -0800 (PST) Received: from mail-pd0-x22a.google.com (mail-pd0-x22a.google.com [2607:f8b0:400e:c02::22a]) by mx.google.com with ESMTPS id t39si22920586yhp.25.2014.01.13.17.52.57 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Mon, 13 Jan 2014 17:52:57 -0800 (PST) Received: by mail-pd0-f170.google.com with SMTP id z10so2837469pdj.29 for ; Mon, 13 Jan 2014 17:52:56 -0800 (PST) Date: Mon, 13 Jan 2014 17:52:30 -0800 (PST) From: Hugh Dickins Subject: [PATCH 2/3] mm/memcg: fix endless iteration in reclaim In-Reply-To: Message-ID: References: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: Johannes Weiner , Andrew Morton , linux-mm@kvack.org, 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(); -- 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