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

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Date:   Thu, 03 May 2018 09:39:03 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Kirill Tkhai <ktkhai@...tuozzo.com>, akpm@...ux-foundation.org,
        peterz@...radead.org, viro@...iv.linux.org.uk, mingo@...nel.org,
        paulmck@...ux.vnet.ibm.com, keescook@...omium.org, riel@...hat.com,
        tglx@...utronix.de, kirill.shutemov@...ux.intel.com,
        marcos.souza.org@...il.com, hoeun.ryu@...il.com,
        pasha.tatashin@...cle.com, gs051095@...il.com, dhowells@...hat.com,
        rppt@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
        Balbir Singh <balbir@...ux.vnet.ibm.com>,
        Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH] memcg: Replace mm->owner with mm->memcg

Oleg Nesterov <oleg@...hat.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