[Devel] Re: [PATCH v3 16/28] memcg: kmem controller charge/uncharge infrastructure

[Glauber Costa]

On 05/30/2012 05:53 PM, Frederic Weisbecker wrote:
> On Wed, May 30, 2012 at 05:37:57PM +0400, Glauber Costa wrote:
>> On 05/30/2012 05:37 PM, Frederic Weisbecker wrote:
>>> Right. __mem_cgroup_get_kmem_cache() fetches the memcg of the owner
>>> and calls memcg_create_cache_enqueue() which does css_tryget(&memcg->css).
>>> After this tryget I think you're fine. And in-between you're safe against
>>> css_set removal due to rcu_read_lock().
>>>
>>> I'm less clear with __mem_cgroup_new_kmem_page() though...
>>
>> That one does not get memcg->css but it does call mem_cgroup_get(),
>> that does prevent against the memcg structure being freed, which I
>> believe to be good enough.
>
> What if the owner calls cgroup_exit() between mem_cgroup_from_task()
> and mem_cgroup_get()? The css_set which contains the memcg gets freed.
> Also the reference on the memcg doesn't even prevent the css_set to
> be removed, does it?
It doesn't, but we don't really care. The css can go away, if the memcg 
structure stays. The caches will outlive the memcg anyway, since it is 
possible that you delete it, with some caches still holding objects that
are not freed (they will be marked as dead).