mptcp: attempt coalescing when moving skbs to mptcp rx queue

We can try to coalesce skbs we take from the subflows rx queue with the
tail of the mptcp rx queue.

If successful, the skb head can be discarded early.

We can also free the skb extensions, we do not access them after this.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/protocol.c

@@ -144,12 +144,29 @@ static void __mptcp_move_skb(struct mptcp_sock *msk, struct sock *ssk,
 			     unsigned int offset, size_t copy_len)
 {
 	struct sock *sk = (struct sock *)msk;
+	struct sk_buff *tail;
 
 	__skb_unlink(skb, &ssk->sk_receive_queue);
-	skb_set_owner_r(skb, sk);
-	__skb_queue_tail(&sk->sk_receive_queue, skb);
 
+	skb_ext_reset(skb);
+	skb_orphan(skb);
 	msk->ack_seq += copy_len;
+
+	tail = skb_peek_tail(&sk->sk_receive_queue);
+	if (offset == 0 && tail) {
+		bool fragstolen;
+		int delta;
+
+		if (skb_try_coalesce(tail, skb, &fragstolen, &delta)) {
+			kfree_skb_partial(skb, fragstolen);
+			atomic_add(delta, &sk->sk_rmem_alloc);
+			sk_mem_charge(sk, delta);
+			return;
+		}
+	}
+
+	skb_set_owner_r(skb, sk);
+	__skb_queue_tail(&sk->sk_receive_queue, skb);
 	MPTCP_SKB_CB(skb)->offset = offset;
 }