Messages in this thread | | | Date | Wed, 30 May 2012 20:16:59 +0400 | From | Glauber Costa <> | Subject | Re: [PATCH v3 16/28] memcg: kmem controller charge/uncharge infrastructure |
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On 05/30/2012 07:33 PM, Frederic Weisbecker wrote: > On Wed, May 30, 2012 at 05:55:38PM +0400, Glauber Costa wrote: >> 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. > > Ah right, the memcg itself is only freed at destroy time. > >> 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). > > I guess I need to look at how the destroy path is handled in your patchset > then. Or how you ensure that __mem_cgroup_new_kmem_page() can't race against > destroy. Appreciate that, thanks.
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