[Devel] Re: [PATCH v3 06/13] memcg: kmem controller infrastructure

[Michal Hocko]

On Fri 28-09-12 15:34:19, Glauber Costa wrote:
> On 09/27/2012 05:44 PM, Michal Hocko wrote:
> >> > the reference count aquired by mem_cgroup_get will still prevent the
> >> > memcg from going away, no?
> > Yes but you are outside of the rcu now and we usually do css_get before
> > we rcu_unlock. mem_cgroup_get just makes sure the group doesn't get
> > deallocated but it could be gone before you call it. Or I am just
> > confused - these 2 levels of ref counting is really not nice.
> > 
> > Anyway, I have just noticed that __mem_cgroup_try_charge does
> > VM_BUG_ON(css_is_removed(&memcg->css)) on a given memcg so you should
> > keep css ref count up as well.
> > 
> 
> IIRC, css_get will prevent the cgroup directory from being removed.
> Because some allocations are expected to outlive the cgroup, we
> specifically don't want that.

Yes, but how do you guarantee that the above VM_BUG_ON doesn't trigger?
Task could have been moved to another group between mem_cgroup_from_task
and mem_cgroup_get, no?
-- 
Michal Hocko
SUSE Labs