Re: [PATCH 2/3] mm/memcg: fix endless iteration in reclaim

[Michal Hocko <mhocko@suse.cz>]

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 <hughd@google.com>
> > ---
> > 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
> 
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-- 
Michal Hocko
SUSE Labs

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