Re: [PATCH v2] net: memcg: late association of sock to memcg

[Shakeel Butt <shakeelb@google.com>]

On Wed, Mar 4, 2020 at 8:38 PM Eric Dumazet <edumazet@google.com> wrote:
>
> On Wed, Mar 4, 2020 at 6:19 PM Shakeel Butt <shakeelb@google.com> wrote:
> >
> > On Wed, Mar 4, 2020 at 5:36 PM Eric Dumazet <edumazet@google.com> wrote:
> > >
> > > On Wed, Mar 4, 2020 at 3:39 PM Shakeel Butt <shakeelb@google.com> wrote:
> > > >
> > > > If a TCP socket is allocated in IRQ context or cloned from unassociated
> > > > (i.e. not associated to a memcg) in IRQ context then it will remain
> > > > unassociated for its whole life. Almost half of the TCPs created on the
> > > > system are created in IRQ context, so, memory used by such sockets will
> > > > not be accounted by the memcg.
> > > >
> > > > This issue is more widespread in cgroup v1 where network memory
> > > > accounting is opt-in but it can happen in cgroup v2 if the source socket
> > > > for the cloning was created in root memcg.
> > > >
> > > > To fix the issue, just do the late association of the unassociated
> > > > sockets at accept() time in the process context and then force charge
> > > > the memory buffer already reserved by the socket.
> > > >
> > > > Signed-off-by: Shakeel Butt <shakeelb@google.com>
> > > > ---
> > > > Changes since v1:
> > > > - added sk->sk_rmem_alloc to initial charging.
> > > > - added synchronization to get memory usage and set sk_memcg race-free.
> > > >
> > > >  net/ipv4/inet_connection_sock.c | 19 +++++++++++++++++++
> > > >  1 file changed, 19 insertions(+)
> > > >
> > > > diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
> > > > index a4db79b1b643..7bcd657cd45e 100644
> > > > --- a/net/ipv4/inet_connection_sock.c
> > > > +++ b/net/ipv4/inet_connection_sock.c
> > > > @@ -482,6 +482,25 @@ struct sock *inet_csk_accept(struct sock *sk, int flags, int *err, bool kern)
> > > >                 }
> > > >                 spin_unlock_bh(&queue->fastopenq.lock);
> > > >         }
> > > > +
> > > > +       if (mem_cgroup_sockets_enabled && !newsk->sk_memcg) {
> > > > +               int amt;
> > > > +
> > > > +               /* atomically get the memory usage and set sk->sk_memcg. */
> > > > +               lock_sock(newsk);
> > > > +
> > > > +               /* The sk has not been accepted yet, no need to look at
> > > > +                * sk->sk_wmem_queued.
> > > > +                */
> > > > +               amt = sk_mem_pages(newsk->sk_forward_alloc +
> > > > +                                  atomic_read(&sk->sk_rmem_alloc));
> > > > +               mem_cgroup_sk_alloc(newsk);
> > > > +
> > > > +               release_sock(newsk);
> > > > +
> > > > +               if (newsk->sk_memcg)
> > >
> > > Most sockets in accept queue should have amt == 0, so maybe avoid
> > > calling this thing only when amt == 0 ?
> > >
> >
> > Thanks, will do in the next version. BTW I have tested with adding
> > mdelay() here and running iperf3 and I did see non-zero amt.
> >
> > > Also  I would release_sock(newsk) after this, otherwise incoming
> > > packets could mess with newsk->sk_forward_alloc
> > >
> >
> > I think that is fine. Once sk->sk_memcg is set then
> > mem_cgroup_charge_skmem() will be called for new incoming packets.
> > Here we just need to call mem_cgroup_charge_skmem() with amt before
> > sk->sk_memcg was set.
>
>
> Unfortunately, as soon as release_sock(newsk) is done, incoming
> packets can be fed to the socket,
> and completely change memory usage of the socket.
>
> For example, the whole queue might have been zapped, or collapsed, if
> we receive a RST packet,
> or if memory pressure asks us to prune the out of order queue.
>
> So you might charge something, then never uncharge it, since at
> close() time the socket will have zero bytes to uncharge.
>

Ok, thanks for the explanation. I will fix this in the next version.