Currently the MPTCP core enforces that when MPTCP-level retrans timer
fires, at most a single dfrag is retransmitted. If some corner-cases it
may be necessary retransmit multiple dfrags, and the MPTCP socket will
need to wait multiple retrans timeout to accomplish that.

Remove the mentioned constraint, allowing to transmit multiple dfrags per
retrans period, as long as the scheduler keeps selecting subflows for
retransmissions and pending data is available in the rtx queue.
The default scheduler will transmit a dfrag per available subflow.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
 net/mptcp/protocol.c | 84 +++++++++++++++++++++++++-------------------
 1 file changed, 47 insertions(+), 37 deletions(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 093c50a43bcb..da84bf5410da 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -1187,13 +1187,6 @@ static void __mptcp_clean_una_wakeup(struct sock *sk)
 	mptcp_write_space(sk);
 }
 
-static void mptcp_clean_una_wakeup(struct sock *sk)
-{
-	mptcp_data_lock(sk);
-	__mptcp_clean_una_wakeup(sk);
-	mptcp_data_unlock(sk);
-}
-
 static void mptcp_enter_memory_pressure(struct sock *sk)
 {
 	struct mptcp_subflow_context *subflow;
@@ -2820,8 +2813,12 @@ static void mptcp_check_fastclose(struct mptcp_sock *msk)
 	sk_error_report(sk);
 }
 
-/* Retransmit the specified data fragment on all the selected subflows. */
-static int __mptcp_push_retrans(struct sock *sk, struct mptcp_data_frag *dfrag)
+/*
+ * Retransmit the specified data fragment on all the selected subflows,
+ * starting from the specified sequence
+ */
+static int __mptcp_push_retrans(struct sock *sk, struct mptcp_data_frag *dfrag,
+				u64 retrans_seq)
 {
 	struct mptcp_sendmsg_info info = { .data_lock_held = true, };
 	struct mptcp_sock *msk = mptcp_sk(sk);
@@ -2840,7 +2837,7 @@ static int __mptcp_push_retrans(struct sock *sk, struct mptcp_data_frag *dfrag)
 			lock_sock(ssk);
 
 			/* limit retransmission to the bytes already sent on some subflows */
-			info.sent = 0;
+			info.sent = retrans_seq - dfrag->data_seq;
 			info.limit = READ_ONCE(msk->csum_enabled) ? dfrag->data_len :
 								    dfrag->already_sent;
 
@@ -2886,42 +2883,55 @@ static void __mptcp_retrans(struct sock *sk)
 	struct mptcp_sock *msk = mptcp_sk(sk);
 	struct mptcp_subflow_context *subflow;
 	struct mptcp_data_frag *dfrag;
+	u64 retrans_seq;
 	int err, len;
 
-	mptcp_clean_una_wakeup(sk);
-
-	/* first check ssk: need to kick "stale" logic */
-	err = mptcp_sched_get_retrans(msk);
-	dfrag = mptcp_rtx_head(sk);
-	if (!dfrag) {
-		if (mptcp_data_fin_enabled(msk)) {
-			struct inet_connection_sock *icsk = inet_csk(sk);
+	mptcp_data_lock(sk);
+	__mptcp_clean_una_wakeup(sk);
+	retrans_seq = msk->snd_una;
+	mptcp_data_unlock(sk);
 
-			WRITE_ONCE(icsk->icsk_retransmits,
-				   icsk->icsk_retransmits + 1);
-			mptcp_set_datafin_timeout(sk);
-			mptcp_send_ack(msk);
+	for (;;) {
+		/* first check ssk: need to kick "stale" logic */
+		err = mptcp_sched_get_retrans(msk);
+		dfrag = mptcp_rtx_head(sk);
+		if (!dfrag) {
+			if (mptcp_data_fin_enabled(msk)) {
+				struct inet_connection_sock *icsk = inet_csk(sk);
+
+				WRITE_ONCE(icsk->icsk_retransmits,
+					   icsk->icsk_retransmits + 1);
+				mptcp_set_datafin_timeout(sk);
+				mptcp_send_ack(msk);
+				break;
+			}
 
-			goto reset_timer;
+			if (!mptcp_send_head(sk))
+				goto clear_scheduled;
+			break;
 		}
 
-		if (!mptcp_send_head(sk))
-			goto clear_scheduled;
-
-		goto reset_timer;
-	}
-
-	if (err)
-		goto reset_timer;
+		if (err)
+			break;
 
-	len = __mptcp_push_retrans(sk, dfrag);
-	if (len < 0)
-		goto clear_scheduled;
+		/* Skip the data already retransmitted in this run */
+		while (dfrag && !before64(retrans_seq, dfrag->data_seq +
+					  dfrag->already_sent))
+			dfrag = list_is_last(&dfrag->list, &msk->rtx_queue) ? NULL :
+				list_next_entry(dfrag, list);
+		if (!dfrag || !dfrag->already_sent)
+			break;
 
-	msk->bytes_retrans += len;
-	dfrag->already_sent = max(dfrag->already_sent, len);
+		len = __mptcp_push_retrans(sk, dfrag, retrans_seq);
+		if (len < 0)
+			goto clear_scheduled;
+		if (!len)
+			break;
 
-reset_timer:
+		retrans_seq += len;
+		msk->bytes_retrans += len;
+		dfrag->already_sent = max(dfrag->already_sent, len);
+	}
 	mptcp_check_and_set_pending(sk);
 
 	if (!mptcp_rtx_timer_pending(sk))
-- 
2.53.0