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 XXXXXXX..XXXXXXX 100644 --- a/net/mptcp/protocol.c +++ b/net/mptcp/protocol.c @@ -XXX,XX +XXX,XX @@ 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 From: Gang Yan <yangang@kylinos.cn> commit fcf04b14334641f4b0b8647824480935e9416d52 upstream. On passive MPTCP connections, the msk sndbuf is not updated correctly. The root cause is an order issue in the accept path: - tcp_check_req() -> subflow_syn_recv_sock() -> mptcp_sk_clone_init() calls __mptcp_propagate_sndbuf() to copy the ssk sndbuf into msk - Later, tcp_child_process() -> tcp_init_transfer() -> tcp_sndbuf_expand() grows the ssk sndbuf. So __mptcp_propagate_sndbuf() runs before the ssk sndbuf has been expanded and the msk ends up with a much smaller sndbuf than the subflow: MPTCP: msk->sndbuf:20480, msk->first->sndbuf:2626560 Fix this by moving the __mptcp_propagate_sndbuf() call from mptcp_sk_clone_init() -- the ssk sndbuf is not yet finalized there -- to __mptcp_propagate_sndbuf() at accept() time, when the ssk sndbuf has been fully expanded by tcp_sndbuf_expand(). Fixes: 8005184fd1ca ("mptcp: refactor sndbuf auto-tuning") Cc: stable@vger.kernel.org Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/602 Signed-off-by: Gang Yan <yangang@kylinos.cn> Acked-by: Paolo Abeni <pabeni@redhat.com> Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org> Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org> Link: https://patch.msgid.link/20260420-net-mptcp-sync-sndbuf-accept-v1-1-e3523e3aeb44@kernel.org Signed-off-by: Paolo Abeni <pabeni@redhat.com> [ No conflicts, but move __mptcp_propagate_sndbuf() above the for-loop (mptcp_for_each_subflow()) present in this version, which will modify 'subflow' used by __mptcp_propagate_sndbuf() in this new patch. ] Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org> --- net/mptcp/protocol.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c index XXXXXXX..XXXXXXX 100644 --- a/net/mptcp/protocol.c +++ b/net/mptcp/protocol.c @@ -XXX,XX +XXX,XX @@ struct sock *mptcp_sk_clone_init(const struct sock *sk, * uses the correct data */ mptcp_copy_inaddrs(nsk, ssk); - __mptcp_propagate_sndbuf(nsk, ssk); mptcp_rcv_space_init(msk, ssk); msk->rcvq_space.time = mptcp_stamp(); @@ -XXX,XX +XXX,XX @@ static int mptcp_stream_accept(struct socket *sock, struct socket *newsock, msk = mptcp_sk(newsk); msk->in_accept_queue = 0; + __mptcp_propagate_sndbuf(newsk, mptcp_subflow_tcp_sock(subflow)); + /* set ssk->sk_socket of accept()ed flows to mptcp socket. * This is needed so NOSPACE flag can be set from tcp stack. */ -- 2.53.0