mptcp: track and update contiguous data status

This patch adds a new member allow_infinite_fallback in mptcp_sock,
which is initialized to 'true' when the connection begins and is set
to 'false' on any retransmit or successful MP_JOIN. Only do infinite
mapping fallback if there is a single subflow AND there have been no
retransmissions AND there have never been any MP_JOINs.

Suggested-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/protocol.c

@@ -2465,6 +2465,7 @@ static void __mptcp_retrans(struct sock *sk)
 		dfrag->already_sent = max(dfrag->already_sent, info.sent);
 		tcp_push(ssk, 0, info.mss_now, tcp_sk(ssk)->nonagle,
 			 info.size_goal);
+		WRITE_ONCE(msk->allow_infinite_fallback, false);
 	}
 
 	release_sock(ssk);
@@ -2539,6 +2540,7 @@ static int __mptcp_init_sock(struct sock *sk)
 	msk->first = NULL;
 	inet_csk(sk)->icsk_sync_mss = mptcp_sync_mss;
 	WRITE_ONCE(msk->csum_enabled, mptcp_is_checksum_enabled(sock_net(sk)));
+	WRITE_ONCE(msk->allow_infinite_fallback, true);
 	msk->recovery = false;
 
 	mptcp_pm_data_init(msk);
@@ -3275,6 +3277,7 @@ bool mptcp_finish_join(struct sock *ssk)
 	}
 
 	subflow->map_seq = READ_ONCE(msk->ack_seq);
+	WRITE_ONCE(msk->allow_infinite_fallback, false);
 
 out:
 	mptcp_event(MPTCP_EVENT_SUB_ESTABLISHED, msk, ssk, GFP_ATOMIC);

net/mptcp/protocol.h

@@ -263,6 +263,7 @@ struct mptcp_sock {
 	bool		rcv_fastclose;
 	bool		use_64bit_ack; /* Set when we received a 64-bit DSN */
 	bool		csum_enabled;
+	bool		allow_infinite_fallback;
 	u8		recvmsg_inq:1,
 			cork:1,
 			nodelay:1;

net/mptcp/subflow.c

@@ -1206,7 +1206,8 @@ static bool subflow_check_data_avail(struct sock *ssk)
 	if (!__mptcp_check_fallback(msk)) {
 		/* RFC 8684 section 3.7. */
 		if (subflow->send_mp_fail) {
-			if (mptcp_has_another_subflow(ssk)) {
+			if (mptcp_has_another_subflow(ssk) ||
+			    !READ_ONCE(msk->allow_infinite_fallback)) {
 				ssk->sk_err = EBADMSG;
 				tcp_set_state(ssk, TCP_CLOSE);
 				subflow->reset_transient = 0;
@@ -1486,6 +1487,7 @@ int __mptcp_subflow_connect(struct sock *sk, const struct mptcp_addr_info *loc,
 	/* discard the subflow socket */
 	mptcp_sock_graft(ssk, sk->sk_socket);
 	iput(SOCK_INODE(sf));
+	WRITE_ONCE(msk->allow_infinite_fallback, false);
 	return err;
 
 failed_unlink: