Re: [PATCH net] Revert "defer call to mem_cgroup_sk_alloc()"

[Roman Gushchin]

On Fri, Feb 02, 2018 at 09:59:27AM -0800, Eric Dumazet wrote:
> On Fri, 2018-02-02 at 16:57 +0000, Roman Gushchin wrote:
> > This patch effectively reverts commit 9f1c2674b328 ("net: memcontrol:
> > defer call to mem_cgroup_sk_alloc()").
> > 
> > Moving mem_cgroup_sk_alloc() to the inet_csk_accept() completely breaks
> > memcg socket memory accounting, as packets received before memcg
> > pointer initialization are not accounted and are causing refcounting
> > underflow on socket release.
> > 
> > Actually the free-after-use problem was fixed by
> > commit c0576e397508 ("net: call cgroup_sk_alloc() earlier in
> > sk_clone_lock()") for the cgroup pointer.
> > 
> > So, let's revert it and call mem_cgroup_sk_alloc() just before
> > cgroup_sk_alloc(). This is safe, as we hold a reference to the socket
> > we're cloning, and it holds a reference to the memcg.
> > 
> > Signed-off-by: Roman Gushchin <guro@xxxxxx>
> > Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
> > Cc: David S. Miller <davem@xxxxxxxxxxxxx>
> > Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> > Cc: Tejun Heo <tj@xxxxxxxxxx>
> > ---
> >  mm/memcontrol.c                 | 14 ++++++++++++++
> >  net/core/sock.c                 |  5 +----
> >  net/ipv4/inet_connection_sock.c |  1 -
> >  3 files changed, 15 insertions(+), 5 deletions(-)
> > 
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 0ae2dc3a1748..0937f2c52c7d 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -5747,6 +5747,20 @@ void mem_cgroup_sk_alloc(struct sock *sk)
> >  	if (!mem_cgroup_sockets_enabled)
> >  		return;
> >  
> > +	/*
> > +	 * Socket cloning can throw us here with sk_memcg already
> > +	 * filled. It won't however, necessarily happen from
> > +	 * process context. So the test for root memcg given
> > +	 * the current task's memcg won't help us in this case.
> > +	 *
> > +	 * Respecting the original socket's memcg is a better
> > +	 * decision in this case.
> > +	 */
> > +	if (sk->sk_memcg) {
> 
> Original commit had a BUG_ON(mem_cgroup_is_root(sk->sk_memcg));
> 
> I presume it is no longer useful ?

Idk, how even we can hit it? And if so, what scary will happen?

If you prefer to have it there, I definitely can return it,
but I see no profit so far.

Thanks!