Re: [RFC][PATCH 1/7] memcg: use css_get in sock_update_memcg()

[Michal Hocko]

On Wed 03-04-13 16:58:48, Glauber Costa wrote:
> On 04/03/2013 01:11 PM, Li Zefan wrote:
> > Use css_get/css_put instead of mem_cgroup_get/put.
> > 
> > Note, if at the same time someone is moving @current to a different
> > cgroup and removing the old cgroup, css_tryget() may return false,
> > and sock->sk_cgrp won't be initialized.
> > 
> > Signed-off-by: Li Zefan <lizefan@xxxxxxxxxx>
> > ---
> >  mm/memcontrol.c | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> > 
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 23d0f6e..43ca91d 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -536,15 +536,15 @@ void sock_update_memcg(struct sock *sk)
> >  		 */
> >  		if (sk->sk_cgrp) {
> >  			BUG_ON(mem_cgroup_is_root(sk->sk_cgrp->memcg));
> > -			mem_cgroup_get(sk->sk_cgrp->memcg);
> > +			css_get(&sk->sk_cgrp->memcg->css);

I am not sure I understand this one. So we have a goup here (which means
that somebody already took a reference on it, right?) and we are taking
another reference. If this is released by sock_release_memcg then who
releases the previous one? It is not directly related to the patch
because this has been done previously already. Could you clarify
Glauber, please?

> >  			return;
> >  		}
> >  
> >  		rcu_read_lock();
> >  		memcg = mem_cgroup_from_task(current);
> >  		cg_proto = sk->sk_prot->proto_cgroup(memcg);
> > -		if (!mem_cgroup_is_root(memcg) && memcg_proto_active(cg_proto)) {
> > -			mem_cgroup_get(memcg);
> > +		if (!mem_cgroup_is_root(memcg) &&
> > +		    memcg_proto_active(cg_proto) && css_tryget(&memcg->css)) {
> >  			sk->sk_cgrp = cg_proto;
> >  		}
> 
> What happens if this tryget fails ? Won't we leak a reference here? We
> will put regardless when the socket is released, and this may go
> negative. No?
 
AFAICS sock_release_memcg releases the reference only if sk->sk_cgrp and
that one wouldn't be set if css_tryget fails.

-- 
Michal Hocko
SUSE Labs
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