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

[Glauber Costa]

On 10/01/2012 03:58 PM, Michal Hocko wrote:
> On Mon 01-10-12 15:51:20, Glauber Costa wrote:
>> 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.
> 
> That is correct but it assumes that the css is valid so somebody upwards
> made sure css will not go away. This would suggest css_get is not
> necessary here but I guess the primary intention here is to make the
> code easier so that we do not have to check whether we took css
> reference on the return path.
> 
In any case, umem would also suffer from double reference, so I'm fine
taking it here as well, since a solution for that is orthogonal.

I still need mem_cgroup_get() to make sure the data structure stays
around, but we only need to do it once at first charge.