Re: [PATCH] memcg: Replace mm->owner with mm->memcg

[ebiederm@xmission.com (Eric W. Biederman)]

Oleg Nesterov <oleg@redhat.com> writes:

> On 05/02, Eric W. Biederman wrote:
>>
>> +static void mem_cgroup_fork(struct task_struct *tsk)
>> +{
>> +	struct cgroup_subsys_state *css;
>> +
>> +	rcu_read_lock();
>> +	css = task_css(tsk, memory_cgrp_id);
>> +	if (css && css_tryget(css))
>> +		task_update_memcg(tsk, mem_cgroup_from_css(css));
>> +	rcu_read_unlock();
>> +}
>
> Why do we need it?
>
> The child's mm->memcg was already initialized by mm_init_memcg() and we can't
> race with migrate until cgroup_threadgroup_change_end() ?

I admit I missed the cgroup_threadgroup_change_begin
cgroup_threadgroup_change_end pair in fs fork.  In this case it doesn't
matter because mm_init_memcg is called from:

   copy_mm
      dup_mm
        mm_init

And copy_mm is called before we call cgroup_threadgroup_change_begin.
So the race remains.

We could move move cgroup_threadgroup_change_begin earlier, to remove
the need for mem_cgroup_fork.  But I have not analyzed that.

Eric