Re: [PATCH net-next v2 1/3] net: tcp: send zero-window ACK when no memory

[Eric Dumazet]

On Mon, Aug 7, 2023 at 3:47 PM <menglong8.dong@xxxxxxxxx> wrote:
>
> From: Menglong Dong <imagedong@xxxxxxxxxxx>
>
> 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@xxxxxxxxxxx>
> ---
> v2:
> - send 0 rwin ACK for the receive queue empty case when necessary
> - send the ACK immediately by using the ICSK_ACK_NOW flag
> ---
>  include/net/inet_connection_sock.h |  3 ++-
>  net/ipv4/tcp_input.c               | 14 +++++++++++---
>  net/ipv4/tcp_output.c              | 14 +++++++++++---
>  3 files changed, 24 insertions(+), 7 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 8e96ebe373d7..aae485d0a3b6 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -5059,12 +5059,20 @@ static void tcp_data_queue(struct sock *sk, struct sk_buff *skb)
>
>                 /* Ok. In sequence. In window. */
>  queue_and_out:
> -               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)) {
> +               if (skb_queue_len(&sk->sk_receive_queue) == 0) {
> +                       if (tcp_try_rmem_schedule(sk, skb, skb->truesize)) {
> +                               sk_forced_mem_schedule(sk, skb->truesize);

I think we want sk->sk_data_ready() here, to let applications drain the queue,
regardless of sk->sk_rcvlowat value.


> +                               inet_csk(sk)->icsk_ack.pending |=
> +                                       (ICSK_ACK_NOMEM | ICSK_ACK_NOW);
> +                               inet_csk_schedule_ack(sk);
> +                       }
> +               } else if (tcp_try_rmem_schedule(sk, skb, skb->truesize)) {
>                         reason = SKB_DROP_REASON_PROTO_MEM;
>                         NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPRCVQDROP);
>                         sk->sk_data_ready(sk);

We also want to keep this sk->sk_data_ready(sk) call.

> +                       inet_csk(sk)->icsk_ack.pending |=
> +                               (ICSK_ACK_NOMEM | ICSK_ACK_NOW);
> +                       inet_csk_schedule_ack(sk);
>                         goto drop;
>                 }

This would suggest a different code refactoring, to avoid code duplication.

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 57c8af1859c16eba5e952a23ea959b628006f9c1..dde6c44f2c1e33dcf60c23b49cd99f270874ca96
100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -5050,13 +5050,17 @@ static void tcp_data_queue(struct sock *sk,
struct sk_buff *skb)

                /* Ok. In sequence. In window. */
 queue_and_out:
-               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)) {
-                       reason = SKB_DROP_REASON_PROTO_MEM;
-                       NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPRCVQDROP);
+               if (tcp_try_rmem_schedule(sk, skb, skb->truesize)) {
+                       /* TODO: maybe ratelimit these WIN 0 ACK ? */
+                       inet_csk(sk)->icsk_ack.pending |=
CSK_ACK_NOMEM | ICSK_ACK_NOW;
+                       inet_csk_schedule_ack(sk);
                        sk->sk_data_ready(sk);
-                       goto drop;
+                       if (skb_queue_len(&sk->sk_receive_queue)) {
+                               reason = SKB_DROP_REASON_PROTO_MEM;
+                               NET_INC_STATS(sock_net(sk),
LINUX_MIB_TCPRCVQDROP);
+                               goto drop;
+                       }
+                       sk_forced_mem_schedule(sk, skb->truesize);
                }

                eaten = tcp_queue_rcv(sk, skb, &fragstolen);