Re: [PATCH memcg RFC] net: set proper memcg for net_init hooks allocations

[Vasily Averin <vvs@openvz.org>]

On 4/22/22 23:01, Vasily Averin wrote:
> On 4/21/22 18:56, Shakeel Butt wrote:
>> On Sat, Apr 16, 2022 at 11:39 PM Vasily Averin <vvs@openvz.org> wrote:
>>> @@ -1147,7 +1148,13 @@ static int __register_pernet_operations(struct list_head *list,
>>>                  * setup_net() and cleanup_net() are not possible.
>>>                  */
>>>                 for_each_net(net) {
>>> +                       struct mem_cgroup *old, *memcg = NULL;
>>> +#ifdef CONFIG_MEMCG
>>> +                       memcg = (net == &init_net) ? root_mem_cgroup : mem_cgroup_from_obj(net);
>>
>> memcg from obj is unstable, so you need a reference on memcg. You can
>> introduce get_mem_cgroup_from_kmem() which works for both
>> MEMCG_DATA_OBJCGS and MEMCG_DATA_KMEM. For uncharged objects (like
>> init_net) it should return NULL.
> 
> Could you please elaborate with more details?
> It seems to me mem_cgroup_from_obj() does everything exactly as you say:
> - for slab objects it returns memcg taken from according slab->memcg_data
> - for ex-slab objects (i.e. page->memcg_data & MEMCG_DATA_OBJCGS)
>     page_memcg_check() returns NULL
> - for kmem objects (i.e. page->memcg_data & MEMCG_DATA_KMEM) 
>     page_memcg_check() returns objcg->memcg
> - in another cases
>     page_memcg_check() returns page->memcg_data,
>     so for uncharged objects like init_net NULL should be returned.
> 
> I can introduce exported get_mem_cgroup_from_kmem(), however it should only
> call mem_cgroup_from_obj(), perhaps under read_rcu_lock/unlock.

I think I finally got your point:
Do you mean I should use css_tryget(&memcg->css) for found memcg,
like get_mem_cgroup_from_mm() does?

Thank you,
	Vasily Averin