Commit 05f76b2d authored Jul 26, 2024 by Subash Abhinov Kasiviswanathan Committed by David S. Miller Jul 29, 2024

tcp: Adjust clamping window for applications specifying SO_RCVBUF

tp->scaling_ratio is not updated based on skb->len/skb->truesize once
SO_RCVBUF is set leading to the maximum window scaling to be 25% of
rcvbuf after
commit dfa2f048 ("tcp: get rid of sysctl_tcp_adv_win_scale")
and 50% of rcvbuf after
commit 697a6c8c ("tcp: increase the default TCP scaling ratio").
50% tries to emulate the behavior of older kernels using
sysctl_tcp_adv_win_scale with default value.

Systems which were using a different values of sysctl_tcp_adv_win_scale
in older kernels ended up seeing reduced download speeds in certain
cases as covered in https://lists.openwall.net/netdev/2024/05/15/13


While the sysctl scheme is no longer acceptable, the value of 50% is
a bit conservative when the skb->len/skb->truesize ratio is later
determined to be ~0.66.

Applications not specifying SO_RCVBUF update the window scaling and
the receiver buffer every time data is copied to userspace. This
computation is now used for applications setting SO_RCVBUF to update
the maximum window scaling while ensuring that the receive buffer
is within the application specified limit.

Fixes: dfa2f048 ("tcp: get rid of sysctl_tcp_adv_win_scale")
Signed-off-by: Sean Tranchetti <quic_stranche@quicinc.com>
Signed-off-by: Subash Abhinov Kasiviswanathan <quic_subashab@quicinc.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent e96a79b1

net/ipv4/tcp_input.c

+16 −7

Original line number	Diff line number	Diff line
		@@ -754,8 +754,7 @@ void tcp_rcv_space_adjust(struct sock *sk)
		* <prev RTT . ><current RTT .. ><next RTT .... >
		*/

		if (READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_moderate_rcvbuf) &&
		!(sk->sk_userlocks & SOCK_RCVBUF_LOCK)) {
		if (READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_moderate_rcvbuf)) {
		u64 rcvwin, grow;
		int rcvbuf;

		@@ -771,6 +770,7 @@ void tcp_rcv_space_adjust(struct sock *sk)

		rcvbuf = min_t(u64, tcp_space_from_win(sk, rcvwin),
		READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_rmem[2]));
		if (!(sk->sk_userlocks & SOCK_RCVBUF_LOCK)) {
		if (rcvbuf > sk->sk_rcvbuf) {
		WRITE_ONCE(sk->sk_rcvbuf, rcvbuf);

		@@ -778,6 +778,15 @@ void tcp_rcv_space_adjust(struct sock *sk)
		WRITE_ONCE(tp->window_clamp,
		tcp_win_from_space(sk, rcvbuf));
		}
		} else {
		/* Make the window clamp follow along while being bounded
		* by SO_RCVBUF.
		*/
		int clamp = tcp_win_from_space(sk, min(rcvbuf, sk->sk_rcvbuf));

		if (clamp > tp->window_clamp)
		WRITE_ONCE(tp->window_clamp, clamp);
		}
		}
		tp->rcvq_space.space = copied;