On Fri 28-09-12 15:34:19, Glauber Costa wrote:

On 09/27/2012 05:44 PM, Michal Hocko wrote:

quoted

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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