From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751548AbaANNfA (ORCPT ); Tue, 14 Jan 2014 08:35:00 -0500 Received: from cantor2.suse.de ([195.135.220.15]:54497 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751318AbaANNe7 (ORCPT ); Tue, 14 Jan 2014 08:34:59 -0500 Date: Tue, 14 Jan 2014 14:34:57 +0100 From: Michal Hocko To: Hugh Dickins Cc: Johannes Weiner , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/3] mm/memcg: fix endless iteration in reclaim Message-ID: <20140114133457.GD32227@dhcp22.suse.cz> References: <20140114132727.GB32227@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140114132727.GB32227@dhcp22.suse.cz> 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 On Tue 14-01-14 14:27:27, Michal Hocko wrote: > 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. And looking closer at it we even do no need to be in reclaim for_each_mem_cgroup_tree is used from more places and that might race as well. > 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 -- Michal Hocko SUSE Labs