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

[Glauber Costa]

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.