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Date:   Fri, 4 May 2018 16:20:57 +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: <20180504142056.GA26151@redhat.com>
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On 05/03, Eric W. Biederman wrote:
>
> 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.

Ah yes, you are right.

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

No, cgroup_threadgroup_change_begin() was called early and this was wrong, see
568ac888215c7fb2fabe8ea739b00ec3c1f5d440. Actually there were more problems, say
copy_net() could deadlock because cleanup_net() does do_wait() with net_mutex held.


OK, what about exec() ? mm_init_memcg() initializes bprm->mm->memcg early in
bprm_mm_init(). What if the execing task migrates before exec_mmap() ?

Oleg.