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

[Eric Dumazet <edumazet@google.com>]

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 ?

Also  I would release_sock(newsk) after this, otherwise incoming
packets could mess with newsk->sk_forward_alloc

if (amt && newsk->sk_memcg)
      mem_cgroup_charge_skmem(newsk->sk_memcg, amt);
release_sock(newsk);

Also, I wonder if     mem_cgroup_charge_skmem() has been used at all
these last four years
on arches with PAGE_SIZE != 4096

( SK_MEM_QUANTUM is not anymore PAGE_SIZE, but 4096)



> +                       mem_cgroup_charge_skmem(newsk->sk_memcg, amt);
> +       }
>  out:
>         release_sock(sk);
>         if (req)
> --
> 2.25.0.265.gbab2e86ba0-goog
>