On Mon, Aug 7, 2023 at 3:47 PM [off-list ref] wrote:

quoted

From: Menglong Dong <redacted>

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 <redacted>
---
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.

quoted

+                               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.

quoted

+                       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);