mptcp: avoid blocking in tcp_sendpages

The transmit loop continues to xmit new data until an
error is returned or all data was transmitted.

For the blocking i/o case, this means that tcp_sendpages() may
block on the subflow until more space becomes available, i.e. we end up
sleeping with the mptcp socket lock held.

Instead we should check if a different subflow is ready to be used.

This restarts the subflow sk lookup when the tx operation succeeded
and the tcp subflow can't accept more data or if tcp_sendpages
indicates -EAGAIN on a blocking mptcp socket.

In case all subflows are busy, the existing logic will wait until
a subflow is ready, releasing the mptcp level socket while doing so.

The mptcp worker already sets DONTWAIT, so no need to make
changes there.

Reported-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>

net/mptcp/protocol.c

@@ -590,7 +590,7 @@ static int mptcp_sendmsg_frag(struct sock *sk, struct sock *ssk,
 	 * access the skb after the sendpages call
 	 */
 	ret = do_tcp_sendpages(ssk, page, offset, psize,
-			       msg->msg_flags | MSG_SENDPAGE_NOTLAST);
+			       msg->msg_flags | MSG_SENDPAGE_NOTLAST | MSG_DONTWAIT);
 	if (ret <= 0)
 		return ret;
 
@@ -714,6 +714,7 @@ static int mptcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 	struct socket *ssock;
 	size_t copied = 0;
 	struct sock *ssk;
+	bool tx_ok;
 	long timeo;
 
 	if (msg->msg_flags & ~(MSG_MORE | MSG_DONTWAIT | MSG_NOSIGNAL))
@@ -738,6 +739,7 @@ static int mptcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 		return ret >= 0 ? ret + copied : (copied ? copied : ret);
 	}
 
+restart:
 	mptcp_clean_una(sk);
 
 	__mptcp_flush_join_list(msk);
@@ -759,11 +761,17 @@ static int mptcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 	pr_debug("conn_list->subflow=%p", ssk);
 
 	lock_sock(ssk);
-	while (msg_data_left(msg)) {
+	tx_ok = msg_data_left(msg);
+	while (tx_ok) {
 		ret = mptcp_sendmsg_frag(sk, ssk, msg, NULL, &timeo, &mss_now,
 					 &size_goal);
-		if (ret < 0)
+		if (ret < 0) {
+			if (ret == -EAGAIN && timeo > 0) {
+				release_sock(ssk);
+				goto restart;
+			}
 			break;
+		}
 		if (ret == 0 && unlikely(__mptcp_needs_tcp_fallback(msk))) {
 			/* Can happen for passive sockets:
 			 * 3WHS negotiated MPTCP, but first packet after is
@@ -777,6 +785,15 @@ static int mptcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 		}
 
 		copied += ret;
+		tx_ok = msg_data_left(msg);
+		if (!tx_ok)
+			break;
+		if (!sk_stream_memory_free(ssk)) {
+			tcp_push(ssk, msg->msg_flags, mss_now,
+				 tcp_sk(ssk)->nonagle, size_goal);
+			release_sock(ssk);
+			goto restart;
+		}
 	}
 
 	mptcp_set_timeout(sk, ssk);