Re: 3.13-rc breaks MEMCG_SWAP

[Michal Hocko <mhocko@suse.cz>]

On Mon 16-12-13 17:41:38, Hugh Dickins wrote:
> On Mon, 16 Dec 2013, Tejun Heo wrote:
> > On Mon, Dec 16, 2013 at 06:19:37PM +0100, Michal Hocko wrote:
> > > I have to think about it some more (the brain is not working anymore
> > > today). But what we really need is that nobody gets the same id while
> > > the css is alive. So css_from_id returning NULL doesn't seem to be
> > > enough.
> > 
> > Oh, I meant whether it's necessary to keep css_from_id() working
> > (ie. doing successful lookups) between offline and release, because
> > that's where lifetimes are coupled.  IOW, if it's enough for cgroup to
> > not recycle the ID until all css's are released && fail css_from_id()
> > lookup after the css is offlined, I can make a five liner quick fix.
> 
> Don't take my word on it, I'm too fuzzy on this: but although it would
> be good to refrain from recycling the ID until all css's are released,
> I believe that it would not be good enough to fail css_from_id() once
> the css is offlined - mem_cgroup_uncharge_swap() needs to uncharge the
> hierarchy of the dead memcg (for example, when tmpfs file is removed).
> 
> Uncharging the dead memcg itself is presumably irrelevant, but it does
> need to locate the right parent to uncharge, and NULL css_from_id()
> would make that impossible. 

Exactly!

-- 
Michal Hocko
SUSE Labs

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