Re: [PATCH v3 2/7] socket: initial cgroup code.

[Greg Thelen]

On Sun, Sep 18, 2011 at 5:56 PM, Glauber Costa <glommer@xxxxxxxxxxxxx> wrote:
> We aim to control the amount of kernel memory pinned at any
> time by tcp sockets. To lay the foundations for this work,
> this patch adds a pointer to the kmem_cgroup to the socket
> structure.
>
> Signed-off-by: Glauber Costa <glommer@xxxxxxxxxxxxx>
> CC: David S. Miller <davem@xxxxxxxxxxxxx>
> CC: Hiroyouki Kamezawa <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> CC: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
...
> +void sock_update_memcg(struct sock *sk)
> +{
> +       /* right now a socket spends its whole life in the same cgroup */
> +       BUG_ON(sk->sk_cgrp);
> +
> +       rcu_read_lock();
> +       sk->sk_cgrp = mem_cgroup_from_task(current);
> +
> +       /*
> +        * We don't need to protect against anything task-related, because
> +        * we are basically stuck with the sock pointer that won't change,
> +        * even if the task that originated the socket changes cgroups.
> +        *
> +        * What we do have to guarantee, is that the chain leading us to
> +        * the top level won't change under our noses. Incrementing the
> +        * reference count via cgroup_exclude_rmdir guarantees that.
> +        */
> +       cgroup_exclude_rmdir(mem_cgroup_css(sk->sk_cgrp));

This grabs a css_get() reference, which prevents rmdir (will return
-EBUSY).  How long is this reference held?  I wonder about the case
where a process creates a socket in memcg M1 and later is moved into
memcg M2.  At that point an admin would expect to be able to 'rmdir
M1'.  I think this rmdir would return -EBUSY and I suspect it would be
difficult for the admin to understand why the rmdir of M1 failed.  It
seems that to rmdir a memcg, an admin would have to kill all processes
that allocated sockets while in M1.  Such processes may not still be
in M1.

> +       rcu_read_unlock();
> +}
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