tcp: Prevent low rmem stalls with SO_RCVLOWAT.

With SO_RCVLOWAT, under memory pressure,
it is possible to enter a state where:

1. We have not received enough bytes to satisfy SO_RCVLOWAT.
2. We have not entered buffer pressure (see tcp_rmem_pressure()).
3. But, we do not have enough buffer space to accept more packets.

In this case, we advertise 0 rwnd (due to #3) but the application does
not drain the receive queue (no wakeup because of #1 and #2) so the
flow stalls.

Modify the heuristic for SO_RCVLOWAT so that, if we are advertising
rwnd<=rcv_mss, force a wakeup to prevent a stall.

Without this patch, setting tcp_rmem to 6143 and disabling TCP
autotune causes a stalled flow. With this patch, no stall occurs. This
is with RPC-style traffic with large messages.

Fixes: 03f45c883c6f ("tcp: avoid extra wakeups for SO_RCVLOWAT users")
Signed-off-by: Arjun Roy <arjunroy@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20201023184709.217614-1-arjunroy.kdev@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index bae4284..b2bc3d7 100644 (file)
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -485,6 +485,8 @@ static inline bool tcp_stream_is_readable(const struct tcp_sock *tp,
                        return true;
                if (tcp_rmem_pressure(sk))
                        return true;
+               if (tcp_receive_window(tp) <= inet_csk(sk)->icsk_ack.rcv_mss)
+                       return true;
        }
        if (sk->sk_prot->stream_memory_read)
                return sk->sk_prot->stream_memory_read(sk);

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index fc44583..389d1b3 100644 (file)
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -4908,7 +4908,8 @@ void tcp_data_ready(struct sock *sk)
        int avail = tp->rcv_nxt - tp->copied_seq;
 
        if (avail < sk->sk_rcvlowat && !tcp_rmem_pressure(sk) &&
-           !sock_flag(sk, SOCK_DONE))
+           !sock_flag(sk, SOCK_DONE) &&
+           tcp_receive_window(tp) > inet_csk(sk)->icsk_ack.rcv_mss)
                return;
 
        sk->sk_data_ready(sk);