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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Kirill Tkhai <ktkhai@virtuozzo.com>, akpm@linux-foundation.org,
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        rppt@linux.vnet.ibm.com, linux-kernel@vger.kernel.org,
        Balbir Singh <balbir@linux.vnet.ibm.com>,
        Tejun Heo <tj@kernel.org>
References: <20180426130700.GP17484@dhcp22.suse.cz>
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        <87o9hz2sw3.fsf@xmission.com> <87h8nr2sa3.fsf_-_@xmission.com>
        <20180502084708.GC26305@dhcp22.suse.cz>
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        <20180503133338.GA23401@redhat.com>
Date:   Thu, 03 May 2018 09:39:03 -0500
In-Reply-To: <20180503133338.GA23401@redhat.com> (Oleg Nesterov's message of
        "Thu, 3 May 2018 15:33:39 +0200")
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Subject: Re: [PATCH] memcg: Replace mm->owner with mm->memcg
Sender: linux-kernel-owner@vger.kernel.org
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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