From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ee0-f42.google.com (mail-ee0-f42.google.com [74.125.83.42]) by kanga.kvack.org (Postfix) with ESMTP id 65A6F6B0031 for ; Tue, 14 Jan 2014 08:27:29 -0500 (EST) Received: by mail-ee0-f42.google.com with SMTP id e49so216234eek.15 for ; Tue, 14 Jan 2014 05:27:28 -0800 (PST) Received: from mx2.suse.de (cantor2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id r41si1134734eem.80.2014.01.14.05.27.28 for (version=TLSv1 cipher=RC4-SHA bits=128/128); Tue, 14 Jan 2014 05:27:28 -0800 (PST) Date: Tue, 14 Jan 2014 14:27:27 +0100 From: Michal Hocko Subject: Re: [PATCH 2/3] mm/memcg: fix endless iteration in reclaim Message-ID: <20140114132727.GB32227@dhcp22.suse.cz> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Hugh Dickins Cc: Johannes Weiner , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org On Mon 13-01-14 17:52:30, Hugh Dickins wrote: > 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. So you had a single memcg (without any children) and a limit-reclaim on it when you removed it, right? This is nasty because __mem_cgroup_iter_next will try to skip it but there is nothing else so it returns NULL. We update iter->generation++ but that doesn't help us as prev = NULL as this is the first iteration so if (prev && reclaim->generation != iter->generation) break out will not help us. You patch will surely help I am just not sure it is the right thing to do. Let me think about this. Anyway very well spotted! > 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(); -- Michal Hocko SUSE Labs -- 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