mptcp: add missing memory scheduling in the rx path

When moving the skbs from the subflow into the msk receive
queue, we must schedule there the required amount of memory.

Try to borrow the required memory from the subflow, if needed,
so that we leverage the existing TCP heuristic.

Fixes: 6771bfd ("mptcp: update mptcp ack sequence from work queue")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Link: https://lore.kernel.org/r/f6143a6193a083574f11b00dbf7b5ad151bc4ff4.1603810630.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/protocol.c

@@ -274,6 +274,15 @@ static bool __mptcp_move_skb(struct mptcp_sock *msk, struct sock *ssk,
 	skb_ext_reset(skb);
 	skb_orphan(skb);
 
+	/* try to fetch required memory from subflow */
+	if (!sk_rmem_schedule(sk, skb, skb->truesize)) {
+		if (ssk->sk_forward_alloc < skb->truesize)
+			goto drop;
+		__sk_mem_reclaim(ssk, skb->truesize);
+		if (!sk_rmem_schedule(sk, skb, skb->truesize))
+			goto drop;
+	}
+
 	/* the skb map_seq accounts for the skb offset:
 	 * mptcp_subflow_get_mapped_dsn() is based on the current tp->copied_seq
 	 * value
@@ -301,6 +310,7 @@ static bool __mptcp_move_skb(struct mptcp_sock *msk, struct sock *ssk,
 	 * will retransmit as needed, if needed.
 	 */
 	MPTCP_INC_STATS(sock_net(sk), MPTCP_MIB_DUPDATA);
+drop:
 	mptcp_drop(sk, skb);
 	return false;
 }