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


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