[PATCH net] tcp: remove 64 KByte limit for initial tp->rcv_wnd value

[Jason Xing <kerneljasonxing@gmail.com>]

From: Jason Xing <kernelxing@tencent.com>

Recently, we had some servers upgraded to the latest kernel and noticed
the indicator from the user side showed worse results than before. It is
caused by the limitation of tp->rcv_wnd.

In 2018 commit a337531b942b ("tcp: up initial rmem to 128KB and SYN rwin
to around 64KB") limited the initial value of tp->rcv_wnd to 65535, most
CDN teams would not benefit from this change because they cannot have a
large window to receive a big packet, which will be slowed down especially
in long RTT. Small rcv_wnd means slow transfer speed, to some extent. It's
the side effect for the latency/time-sensitive users.

To avoid future confusion, current change doesn't affect the initial
receive window on the wire in a SYN or SYN+ACK packet which are set within
65535 bytes according to RFC 7323 also due to the limit in
__tcp_transmit_skb():

    th->window      = htons(min(tp->rcv_wnd, 65535U));

In one word, __tcp_transmit_skb() already ensures that constraint is
respected, no matter how large tp->rcv_wnd is. The change doesn't violate
RFC.

Let me provide one example if with or without the patch:
Before:
client   --- SYN: rwindow=65535 ---> server
client   <--- SYN+ACK: rwindow=65535 ----  server
client   --- ACK: rwindow=65536 ---> server
Note: for the last ACK, the calculation is 512 << 7.

After:
client   --- SYN: rwindow=65535 ---> server
client   <--- SYN+ACK: rwindow=65535 ----  server
client   --- ACK: rwindow=175232 ---> server
Note: I use the following command to make it work:
ip route change default via [ip] dev eth0 metric 100 initrwnd 120
For the last ACK, the calculation is 1369 << 7.

When we apply such a patch, having a large rcv_wnd if the user tweak this
knob can help transfer data more rapidly and save some rtts.

Fixes: a337531b942b ("tcp: up initial rmem to 128KB and SYN rwin to around 64KB")
Signed-off-by: Jason Xing <kernelxing@tencent.com>
---
v1
Link: https://lore.kernel.org/all/20240518025008.70689-1-kerneljasonxing@gmail.com/
1. refine the changelog (Eric)
2. add fixes tag to make sure the fix is backported (Eric)

RFC v2
Link: https://lore.kernel.org/all/20240517085031.18896-1-kerneljasonxing@gmail.com/
1. revise the title and body messages (Neal)
---
 net/ipv4/tcp_output.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 95caf8aaa8be..95618d0e78e4 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -232,7 +232,7 @@ void tcp_select_initial_window(const struct sock *sk, int __space, __u32 mss,
 	if (READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_workaround_signed_windows))
 		(*rcv_wnd) = min(space, MAX_TCP_WINDOW);
 	else
-		(*rcv_wnd) = min_t(u32, space, U16_MAX);
+		(*rcv_wnd) = space;
 
 	if (init_rcv_wnd)
 		*rcv_wnd = min(*rcv_wnd, init_rcv_wnd * mss);
-- 
2.37.3