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Date: Thu, 3 May 2018 15:33:39 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: "Eric W. Biederman" <ebiederm@xmission.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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        Balbir Singh <balbir@linux.vnet.ibm.com>, Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH] memcg: Replace mm->owner with mm->memcg
Message-ID: <20180503133338.GA23401@redhat.com>
References: <20180426130700.GP17484@dhcp22.suse.cz>
 <87efj2q6sq.fsf@xmission.com>
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 <87r2n01q58.fsf@xmission.com>
 <87o9hz2sw3.fsf@xmission.com>
 <87h8nr2sa3.fsf_-_@xmission.com>
 <20180502084708.GC26305@dhcp22.suse.cz>
 <20180502132026.GB16060@cmpxchg.org>
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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() ?

Oleg.