[PATCH net] tcp: tcp_grow_window() needs to respect tcp_space()

[PATCH net] tcp: tcp_grow_window() needs to respect tcp_space()

[Eric Dumazet]

For some reason, tcp_grow_window() correctly tests if enough room
is present before attempting to increase tp->rcv_ssthresh,
but does not prevent it to grow past tcp_space()

This is causing hard to debug issues, like failing
the (__tcp_select_window(sk) >= tp->rcv_wnd) test
in __tcp_ack_snd_check(), causing ACK delays and possibly
slow flows.

Depending on tcp_rmem[2], MTU, skb->len/skb->truesize ratio,
we can see the problem happening on "netperf -t TCP_RR -- -r 2000,2000"
after about 60 round trips, when the active side no longer sends
immediate acks.

This bug predates git history.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Wei Wang <weiwan@google.com>
---
 net/ipv4/tcp_input.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 5def3c48870e17f42ac9424a6ee091ac4824dabc..731d3045b50a0fb9a89c887a154db9a3da8c7ddd 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -402,11 +402,12 @@ static int __tcp_grow_window(const struct sock *sk, const struct sk_buff *skb)
 static void tcp_grow_window(struct sock *sk, const struct sk_buff *skb)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
+	int room;
+
+	room = min_t(int, tp->window_clamp, tcp_space(sk)) - tp->rcv_ssthresh;
 
 	/* Check #1 */
-	if (tp->rcv_ssthresh < tp->window_clamp &&
-	    (int)tp->rcv_ssthresh < tcp_space(sk) &&
-	    !tcp_under_memory_pressure(sk)) {
+	if (room > 0 && !tcp_under_memory_pressure(sk)) {
 		int incr;
 
 		/* Check #2. Increase window, if skb with such overhead
@@ -419,8 +420,7 @@ static void tcp_grow_window(struct sock *sk, const struct sk_buff *skb)
 
 		if (incr) {
 			incr = max_t(int, incr, 2 * skb->len);
-			tp->rcv_ssthresh = min(tp->rcv_ssthresh + incr,
-					       tp->window_clamp);
+			tp->rcv_ssthresh += min(room, incr);
 			inet_csk(sk)->icsk_ack.quick |= 1;
 		}
 	}
-- 
2.21.0.392.gf8f6787159e-goog

Re: [PATCH net] tcp: tcp_grow_window() needs to respect tcp_space()

[David Miller]

From: Eric Dumazet <edumazet@google.com>
Date: Tue, 16 Apr 2019 10:55:20 -0700

> For some reason, tcp_grow_window() correctly tests if enough room
> is present before attempting to increase tp->rcv_ssthresh,
> but does not prevent it to grow past tcp_space()
> 
> This is causing hard to debug issues, like failing
> the (__tcp_select_window(sk) >= tp->rcv_wnd) test
> in __tcp_ack_snd_check(), causing ACK delays and possibly
> slow flows.
> 
> Depending on tcp_rmem[2], MTU, skb->len/skb->truesize ratio,
> we can see the problem happening on "netperf -t TCP_RR -- -r 2000,2000"
> after about 60 round trips, when the active side no longer sends
> immediate acks.
> 
> This bug predates git history.
> 
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
> Acked-by: Neal Cardwell <ncardwell@google.com>
> Acked-by: Wei Wang <weiwan@google.com>

Applied and queued up for -stable, thanks Eric.