Re: [PATCH net-next 1/3] net: tcp: send zero-window ACK when no memory

[Eric Dumazet <edumazet@google.com>]

On Thu, Jul 27, 2023 at 2:51 PM <menglong8.dong@gmail.com> wrote:
>
> From: Menglong Dong <imagedong@tencent.com>
>
> For now, skb will be dropped when no memory, which makes client keep
> retrans util timeout and it's not friendly to the users.
>
> In this patch, we reply an ACK with zero-window in this case to update
> the snd_wnd of the sender to 0. Therefore, the sender won't timeout the
> connection and will probe the zero-window with the retransmits.
>
> Signed-off-by: Menglong Dong <imagedong@tencent.com>
> ---
>  include/net/inet_connection_sock.h |  3 ++-
>  net/ipv4/tcp_input.c               |  4 ++--
>  net/ipv4/tcp_output.c              | 14 +++++++++++---
>  3 files changed, 15 insertions(+), 6 deletions(-)
>
> diff --git a/include/net/inet_connection_sock.h b/include/net/inet_connection_sock.h
> index c2b15f7e5516..be3c858a2ebb 100644
> --- a/include/net/inet_connection_sock.h
> +++ b/include/net/inet_connection_sock.h
> @@ -164,7 +164,8 @@ enum inet_csk_ack_state_t {
>         ICSK_ACK_TIMER  = 2,
>         ICSK_ACK_PUSHED = 4,
>         ICSK_ACK_PUSHED2 = 8,
> -       ICSK_ACK_NOW = 16       /* Send the next ACK immediately (once) */
> +       ICSK_ACK_NOW = 16,      /* Send the next ACK immediately (once) */
> +       ICSK_ACK_NOMEM = 32,
>  };
>
>  void inet_csk_init_xmit_timers(struct sock *sk,
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 3cd92035e090..03111af6115d 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -5061,7 +5061,8 @@ static void tcp_data_queue(struct sock *sk, struct sk_buff *skb)
>                         reason = SKB_DROP_REASON_PROTO_MEM;
>                         NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPRCVQDROP);
>                         sk->sk_data_ready(sk);
> -                       goto drop;
> +                       inet_csk(sk)->icsk_ack.pending |= ICSK_ACK_NOMEM;

Also set ICSK_ACK_NOW ?

We also need to make sure to send an immediate ACK WIN 0 in the case we queued
the skb in an empty receive queue and we were under pressure.

We do not want to have a delayed ACK sending a normal RWIN,
then wait for another packet that we will probably drop.

Look at the code :

if (skb_queue_len(&sk->sk_receive_queue) == 0)
     sk_forced_mem_schedule(sk, skb->truesize);
else if (tcp_try_rmem_schedule(sk, skb, skb->truesize)) {

and refactor it to make sure to set  ICSK_ACK_NOMEM even on the first packet
that bypassed the rmem_schedule().



> +                       goto out_of_window;

Why forcing quickack mode ? Please leave the "goto drop;"

>                 }
>
>                 eaten = tcp_queue_rcv(sk, skb, &fragstolen);
> @@ -5102,7 +5103,6 @@ static void tcp_data_queue(struct sock *sk, struct sk_buff *skb)
>  out_of_window:
>                 tcp_enter_quickack_mode(sk, TCP_MAX_QUICKACKS);
>                 inet_csk_schedule_ack(sk);
> -drop:
>                 tcp_drop_reason(sk, skb, reason);
>                 return;
>         }
>