netdev - [PATCH net-next 8/9] mptcp: ignore notsent_lowat setting at the subflow level

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Message-Id: <20231023-send-net-next-20231023-2-v1-8-9dc60939d371@kernel.org>
Date: Mon, 23 Oct 2023 13:44:41 -0700
From: Mat Martineau <martineau@...nel.org>
To: Matthieu Baerts <matttbe@...nel.org>, 
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>
Cc: netdev@...r.kernel.org, mptcp@...ts.linux.dev, 
 Mat Martineau <martineau@...nel.org>
Subject: [PATCH net-next 8/9] mptcp: ignore notsent_lowat setting at the
 subflow level

From: Paolo Abeni <pabeni@...hat.com>

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@...nel.org>
Signed-off-by: Paolo Abeni <pabeni@...hat.com>
Signed-off-by: Mat Martineau <martineau@...nel.org>
---
 net/mptcp/sockopt.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/mptcp/sockopt.c b/net/mptcp/sockopt.c
index abf0645cb65d..72858d7d8974 100644
--- 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);

-- 
2.41.0