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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,
        peterz@infradead.org, viro@zeniv.linux.org.uk, mingo@kernel.org,
        paulmck@linux.vnet.ibm.com, keescook@chromium.org, riel@redhat.com,
        tglx@linutronix.de, kirill.shutemov@linux.intel.com,
        marcos.souza.org@gmail.com, hoeun.ryu@gmail.com,
        pasha.tatashin@oracle.com, gs051095@gmail.com, dhowells@redhat.com,
        rppt@linux.vnet.ibm.com, linux-kernel@vger.kernel.org,
        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>
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 <20180502084708.GC26305@dhcp22.suse.cz>
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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.