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    SubjectRe: [PATCH v3 06/13] memcg: kmem controller infrastructure
    On 10/01/2012 03:51 PM, Michal Hocko wrote:
    > On Mon 01-10-12 14:09:09, Glauber Costa wrote:
    >> On 10/01/2012 01:48 PM, Michal Hocko wrote:
    >>> 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?
    >>>
    >>
    >> Ok, after reading this again (and again), you seem to be right. It
    >> concerns me, however, that simply getting the css would lead us to a
    >> double get/put pair, since try_charge will have to do it anyway.
    >
    > That happens only for !*ptr case and you provide a memcg here, don't
    > you.
    >

    if (*ptr) { /* css should be a valid one */
    memcg = *ptr;
    VM_BUG_ON(css_is_removed(&memcg->css));
    if (mem_cgroup_is_root(memcg))
    goto done;
    if (consume_stock(memcg, nr_pages))
    goto done;
    css_get(&memcg->css);


    The way I read this, this will still issue a css_get here, unless
    consume_stock suceeds (assuming non-root)

    So we'd still have to have a wrapping get/put pair outside the charge.



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