Re: [PATCH v3] net: memcg: late association of sock to memcg

Eric Dumazet
2020-03-05T21:17:14Z

On 3/5/20 12:55 PM, Shakeel Butt 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 v2:
> - Additional check for charging.
> - Release the sock after charging.
>
> 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 | 20 ++++++++++++++++++++
> 1 file changed, 20 insertions(+)
>
> diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
> index a4db79b1b643..5face55cf818 100644
> --- a/net/ipv4/inet_connection_sock.c
> +++ b/net/ipv4/inet_connection_sock.c
> @@ -482,6 +482,26 @@ 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, set and charge the
> + * 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);
> + if (newsk->sk_memcg && amt)
> + mem_cgroup_charge_skmem(newsk->sk_memcg, amt);
> +
> + release_sock(newsk);
> + }
> out:
> release_sock(sk);
> if (req)
>

This patch looks fine, but why keeping the mem_cgroup_sk_alloc(newsk);
in sk_clone_lock() ?

Note that all TCP sk_clone_lock() calls happen in softirq context.