Re: [PATCH memcg v4] net: set proper memcg for net_init hooks allocations

[Vasily Averin <vvs@openvz.org>]

On 4/27/22 18:06, Shakeel Butt wrote:
> On Wed, Apr 27, 2022 at 5:22 AM Michal Koutný <mkoutny@suse.com> wrote:
>>
>> On Tue, Apr 26, 2022 at 10:23:32PM -0700, Shakeel Butt <shakeelb@google.com> wrote:
>>> [...]
>>>>
>>>> +static inline struct mem_cgroup *get_mem_cgroup_from_obj(void *p)
>>>> +{
>>>> +       struct mem_cgroup *memcg;
>>>> +
>>>
>>> Do we need memcg_kmem_enabled() check here or maybe
>>> mem_cgroup_from_obj() should be doing memcg_kmem_enabled() instead of
>>> mem_cgroup_disabled() as we can have "cgroup.memory=nokmem" boot
>>> param.

Shakeel, unfortunately I'm not ready to answer this question right now.
I even did not noticed that memcg_kmem_enabled() and mem_cgroup_disabled()
have a different nature.
If you have no objections I'm going to keep this place as is and investigate
this question later. 

>> I reckon such a guard is on the charge side and readers should treat
>> NULL and root_mem_group equally. Or is there a case when these two are
>> different?
>>
>> (I can see it's different semantics when stored in current->active_memcg
>> (and active_memcg() getter) but for such "outer" callers like here it
>> seems equal.)

Dear Michal,
I may have misunderstood your point of view, so let me explain my vision
in more detail.
I do not think that NULL and root_mem_cgroup are equal here:
- we have enabled cgroups and well-defined root_mem_cgroup,
- this function is called from inside memcg-limited container,
- we tried to get memcg from net, but without success,
  and as result got NULL from  mem_cgroup_from_obj()
  (frankly speaking I do not think this situation is really possible)
If we keep memcg = NULL, then current's memcg will not be masked and
net_init's allocations will be accounted to current's memcg. 
So we need to set active_memcg to root_mem_cgroup, it helps to avoid
incorrect accounting.

> I was more thinking about possible shortcut optimization and unrelated
> to this patch.
> 
> Vasily, can you please add documentation for get_mem_cgroup_from_obj()
> similar to get_mem_cgroup_from_mm()? Also for mem_cgroup_or_root().
> Please note that root_mem_cgroup can be NULL during early boot.

Ok, thank you for the remark, I'll improve it in next patch version.

Thank you,
	Vasily Averin