linux-kernel - Re: [PATCH v7 00/10] Request for Inclusion: per-cgroup tcp memory pressure

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Date:	Mon, 5 Dec 2011 11:06:19 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Glauber Costa <glommer@...allels.com>
Cc:	<linux-kernel@...r.kernel.org>, <paul@...lmenage.org>,
	<lizf@...fujitsu.com>, <ebiederm@...ssion.com>,
	<davem@...emloft.net>, <gthelen@...gle.com>,
	<netdev@...r.kernel.org>, <linux-mm@...ck.org>,
	<kirill@...temov.name>, <avagin@...allels.com>, <devel@...nvz.org>,
	<eric.dumazet@...il.com>, <cgroups@...r.kernel.org>
Subject: Re: [PATCH v7 00/10] Request for Inclusion: per-cgroup tcp memory
 pressure

On Fri, 2 Dec 2011 16:04:08 -0200
Glauber Costa <glommer@...allels.com> wrote:

> On 11/30/2011 12:11 AM, KAMEZAWA Hiroyuki wrote:
> > On Tue, 29 Nov 2011 21:56:51 -0200
> > Glauber Costa<glommer@...allels.com>  wrote:
> >
> >> Hi,
> >>
> >> This patchset implements per-cgroup tcp memory pressure controls. It did not change
> >> significantly since last submission: rather, it just merges the comments Kame had.
> >> Most of them are style-related and/or Documentation, but there are two real bugs he
> >> managed to spot (thanks)
> >>
> >> Please let me know if there is anything else I should address.
> >>
> >
> > After reading all codes again, I feel some strange. Could you clarify ?
> >
> > Here.
> > ==
> > +void sock_update_memcg(struct sock *sk)
> > +{
> > +	/* right now a socket spends its whole life in the same cgroup */
> > +	if (sk->sk_cgrp) {
> > +		WARN_ON(1);
> > +		return;
> > +	}
> > +	if (static_branch(&memcg_socket_limit_enabled)) {
> > +		struct mem_cgroup *memcg;
> > +
> > +		BUG_ON(!sk->sk_prot->proto_cgroup);
> > +
> > +		rcu_read_lock();
> > +		memcg = mem_cgroup_from_task(current);
> > +		if (!mem_cgroup_is_root(memcg))
> > +			sk->sk_cgrp = sk->sk_prot->proto_cgroup(memcg);
> > +		rcu_read_unlock();
> > ==
> >
> > sk->sk_cgrp is set to a memcg without any reference count.
> >
> > Then, no check for preventing rmdir() and freeing memcgroup.
> >
> > Is there some css_get() or mem_cgroup_get() somewhere ?
> >
> 
> There were a css_get in the first version of this patchset. It was 
> removed, however, because it was deemed anti-intuitive to prevent rmdir, 
> since we can't know which sockets are blocking it, or do anything about 
> it. Or did I misunderstand something ?
> 

Maybe I misuderstood. Thank you. Ok, there is no css_get/put and
rmdir() is allowed. But, hmm....what's guarding threads from stale
pointer access ? 

Does a memory cgroup which is pointed by sk->sk_cgrp always exist ?

-Kame

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