tcp: clamp window like before the cleanup

A recent cleanup changed the behaviour of tcp_set_window_clamp(). This
looks unintentional, and affects MPTCP selftests, e.g. some tests
re-establishing a connection after a disconnect are now unstable.

Before the cleanup, this operation was done:

  new_rcv_ssthresh = min(tp->rcv_wnd, new_window_clamp);
  tp->rcv_ssthresh = max(new_rcv_ssthresh, tp->rcv_ssthresh);

The cleanup used the 'clamp' macro which takes 3 arguments -- value,
lowest, and highest -- and returns a value between the lowest and the
highest allowable values. This then assumes ...

  lowest (rcv_ssthresh) <= highest (rcv_wnd)

... which doesn't seem to be always the case here according to the MPTCP
selftests, even when running them without MPTCP, but only TCP.

For example, when we have ...

  rcv_wnd < rcv_ssthresh < new_rcv_ssthresh

... before the cleanup, the rcv_ssthresh was not changed, while after
the cleanup, it is lowered down to rcv_wnd (highest).

During a simple test with TCP, here are the values I observed:

  new_window_clamp (val)  rcv_ssthresh (lo)  rcv_wnd (hi)
      117760   (out)         65495         <  65536
      128512   (out)         109595        >  80256  => lo > hi
      1184975  (out)         328987        <  329088

      113664   (out)         65483         <  65536
      117760   (out)         110968        <  110976
      129024   (out)         116527        >  109696 => lo > hi

Here, we can see that it is not that rare to have rcv_ssthresh (lo)
higher than rcv_wnd (hi), so having a different behaviour when the
clamp() macro is used, even without MPTCP.

Note: new_window_clamp is always out of range (rcv_ssthresh < rcv_wnd)
here, which seems to be generally the case in my tests with small
connections.

I then suggests reverting this part, not to change the behaviour.

Fixes: 863a952 ("tcp: tcp_set_window_clamp() cleanup")
Closes: multipath-tcp/mptcp_net-next#551
Signed-off-by: Matthieu Baerts (NGI0) <[email protected]>
Tested-by: Jason Xing <[email protected]>
Reviewed-by: Eric Dumazet <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: matttbe

net/ipv4/tcp.c

@@ -3693,7 +3693,7 @@ EXPORT_SYMBOL(tcp_sock_set_keepcnt);
 
 int tcp_set_window_clamp(struct sock *sk, int val)
 {
-	u32 old_window_clamp, new_window_clamp;
+	u32 old_window_clamp, new_window_clamp, new_rcv_ssthresh;
 	struct tcp_sock *tp = tcp_sk(sk);
 
 	if (!val) {
@@ -3714,12 +3714,12 @@ int tcp_set_window_clamp(struct sock *sk, int val)
 	/* Need to apply the reserved mem provisioning only
 	 * when shrinking the window clamp.
 	 */
-	if (new_window_clamp < old_window_clamp)
+	if (new_window_clamp < old_window_clamp) {
 		__tcp_adjust_rcv_ssthresh(sk, new_window_clamp);
-	else
-		tp->rcv_ssthresh = clamp(new_window_clamp,
-					 tp->rcv_ssthresh,
-					 tp->rcv_wnd);
+	} else {
+		new_rcv_ssthresh = min(tp->rcv_wnd, new_window_clamp);
+		tp->rcv_ssthresh = max(new_rcv_ssthresh, tp->rcv_ssthresh);
+	}
 	return 0;
 }