mptcp: ignore notsent_lowat setting at the subflow level

Any latency related tuning taking action at the subflow level does
not really affect the user-space, as only the main MPTCP socket is
relevant.

Anyway any limiting setting may foul the MPTCP scheduler, not being
able to fully use the subflow-level cwin, leading to very poor b/w
usage.

Enforce notsent_lowat to be a no-op on every subflow.

Note that TCP_NOTSENT_LOWAT is currently not supported, and properly
dealing with that will require more invasive changes.

Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20231023-send-net-next-20231023-2-v1-8-9dc60939d371@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/mptcp/sockopt.c b/net/mptcp/sockopt.c
index abf0645cb65d184e8f9d4462e5a1c190c2571e4f..72858d7d8974bc72ed0cc81aebfcb79e2aeb1b7b 100644 (file)
--- a/net/mptcp/sockopt.c
+++ b/net/mptcp/sockopt.c
@@ -1452,6 +1452,12 @@ void mptcp_sockopt_sync_locked(struct mptcp_sock *msk, struct sock *ssk)
 
        ssk->sk_rcvlowat = 0;
 
+       /* subflows must ignore any latency-related settings: will not affect
+        * the user-space - only the msk is relevant - but will foul the
+        * mptcp scheduler
+        */
+       tcp_sk(ssk)->notsent_lowat = UINT_MAX;
+
        if (READ_ONCE(subflow->setsockopt_seq) != msk->setsockopt_seq) {
                sync_socket_options(msk, ssk);