[PATCH mptcp-net] mptcp: sync the msk->sndbuf at accept() time

[Gang Yan <gang.yan@linux.dev>]

From: Gang Yan <yangang@kylinos.cn>

After an MPTCP connection is established, the sk_sndbuf of client's msk
can be updated through 'subflow_finish_connect'. However, the newly
accepted msk on the server side has a small sk_sndbuf than
msk->first->sk_sndbuf:

'''
MPTCP: msk:00000000e55b09db, msk->sndbuf:20480, msk->first->sndbuf:2626560
'''

This means that when the server immediately sends MSG_DONTWAIT data to
the client after the connection is established, it is more likely to
encounter EAGAIN.

This patch synchronizes the sk_sndbuf by triggering its update during accept.

Fixes: 8005184fd1ca ("mptcp: refactor sndbuf auto-tuning")
Signed-off-by: Gang Yan <yangang@kylinos.cn>
---
Notes:

Hi, Matt,

I'm not sure the commit in 'Fix' tag is correct.

The 3a236aef280e (mptcp: refactor passive socket initialization) removes the
'propagate_sndbuf' in '__mptcp_finish_join' and 'mptcp_stream_accept'.
And the 8005184fd1ca adds it in '__mptcp_finish_join' but not in accept
time. So I think there exists a miss in 8005184fd1ca.

Can you help me to check the 'Fix' tag?

Thanks,
Gang
---
 net/mptcp/protocol.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index e484c6391b48..33610d638385 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -4213,6 +4213,7 @@ static int mptcp_stream_accept(struct socket *sock, struct socket *newsock,
 
 		mptcp_graft_subflows(newsk);
 		mptcp_rps_record_subflows(msk);
+		__mptcp_propagate_sndbuf(newsk, subflow->tcp_sock);
 
 		/* Do late cleanup for the first subflow as necessary. Also
 		 * deal with bad peers not doing a complete shutdown.
-- 
2.43.0