tcp: add coalescing attempt in tcp_ofo_queue()

In order to make TCP more resilient in presence of reorders, we need
to allow coalescing to happen when skbs from out of order queue are
transferred into receive queue. LRO/GRO can be completely canceled
in some pathological cases, like per packet load balancing on aggregated
links.

I had to move tcp_try_coalesce() up in the file above tcp_ofo_queue()

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -4061,14 +4061,53 @@ static void tcp_sack_remove(struct tcp_sock *tp)
 	tp->rx_opt.num_sacks = num_sacks;
 }
 
+/**
+ * tcp_try_coalesce - try to merge skb to prior one
+ * @sk: socket
+ * @to: prior buffer
+ * @from: buffer to add in queue
+ * @fragstolen: pointer to boolean
+ *
+ * Before queueing skb @from after @to, try to merge them
+ * to reduce overall memory use and queue lengths, if cost is small.
+ * Packets in ofo or receive queues can stay a long time.
+ * Better try to coalesce them right now to avoid future collapses.
+ * Returns true if caller should free @from instead of queueing it
+ */
+static bool tcp_try_coalesce(struct sock *sk,
+			     struct sk_buff *to,
+			     struct sk_buff *from,
+			     bool *fragstolen)
+{
+	int delta;
+
+	*fragstolen = false;
+
+	/* Its possible this segment overlaps with prior segment in queue */
+	if (TCP_SKB_CB(from)->seq != TCP_SKB_CB(to)->end_seq)
+		return false;
+
+	if (!skb_try_coalesce(to, from, fragstolen, &delta))
+		return false;
+
+	atomic_add(delta, &sk->sk_rmem_alloc);
+	sk_mem_charge(sk, delta);
+	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPRCVCOALESCE);
+	TCP_SKB_CB(to)->end_seq = TCP_SKB_CB(from)->end_seq;
+	TCP_SKB_CB(to)->ack_seq = TCP_SKB_CB(from)->ack_seq;
+	TCP_SKB_CB(to)->tcp_flags |= TCP_SKB_CB(from)->tcp_flags;
+	return true;
+}
+
 /* This one checks to see if we can put data from the
  * out_of_order queue into the receive_queue.
  */
 static void tcp_ofo_queue(struct sock *sk)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	__u32 dsack_high = tp->rcv_nxt;
-	struct sk_buff *skb;
+	struct sk_buff *skb, *tail;
+	bool fragstolen, eaten;
 
 	while ((skb = skb_peek(&tp->out_of_order_queue)) != NULL) {
 		if (after(TCP_SKB_CB(skb)->seq, tp->rcv_nxt))
@@ -4081,21 +4120,25 @@ static void tcp_ofo_queue(struct sock *sk)
 			tcp_dsack_extend(sk, TCP_SKB_CB(skb)->seq, dsack);
 		}
 
+		__skb_unlink(skb, &tp->out_of_order_queue);
 		if (!after(TCP_SKB_CB(skb)->end_seq, tp->rcv_nxt)) {
 			SOCK_DEBUG(sk, "ofo packet was already received\n");
-			__skb_unlink(skb, &tp->out_of_order_queue);
 			__kfree_skb(skb);
 			continue;
 		}
 		SOCK_DEBUG(sk, "ofo requeuing : rcv_next %X seq %X - %X\n",
 			   tp->rcv_nxt, TCP_SKB_CB(skb)->seq,
 			   TCP_SKB_CB(skb)->end_seq);
 
-		__skb_unlink(skb, &tp->out_of_order_queue);
-		__skb_queue_tail(&sk->sk_receive_queue, skb);
+		tail = skb_peek_tail(&sk->sk_receive_queue);
+		eaten = tail && tcp_try_coalesce(sk, tail, skb, &fragstolen);
 		tp->rcv_nxt = TCP_SKB_CB(skb)->end_seq;
+		if (!eaten)
+			__skb_queue_tail(&sk->sk_receive_queue, skb);
 		if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN)
 			tcp_fin(sk);
+		if (eaten)
+			kfree_skb_partial(skb, fragstolen);
 	}
 }
 
@@ -4122,44 +4165,6 @@ static int tcp_try_rmem_schedule(struct sock *sk, struct sk_buff *skb,
 	return 0;
 }
 
-/**
- * tcp_try_coalesce - try to merge skb to prior one
- * @sk: socket
- * @to: prior buffer
- * @from: buffer to add in queue
- * @fragstolen: pointer to boolean
- *
- * Before queueing skb @from after @to, try to merge them
- * to reduce overall memory use and queue lengths, if cost is small.
- * Packets in ofo or receive queues can stay a long time.
- * Better try to coalesce them right now to avoid future collapses.
- * Returns true if caller should free @from instead of queueing it
- */
-static bool tcp_try_coalesce(struct sock *sk,
-			     struct sk_buff *to,
-			     struct sk_buff *from,
-			     bool *fragstolen)
-{
-	int delta;
-
-	*fragstolen = false;
-
-	/* Its possible this segment overlaps with prior segment in queue */
-	if (TCP_SKB_CB(from)->seq != TCP_SKB_CB(to)->end_seq)
-		return false;
-
-	if (!skb_try_coalesce(to, from, fragstolen, &delta))
-		return false;
-
-	atomic_add(delta, &sk->sk_rmem_alloc);
-	sk_mem_charge(sk, delta);
-	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPRCVCOALESCE);
-	TCP_SKB_CB(to)->end_seq = TCP_SKB_CB(from)->end_seq;
-	TCP_SKB_CB(to)->ack_seq = TCP_SKB_CB(from)->ack_seq;
-	TCP_SKB_CB(to)->tcp_flags |= TCP_SKB_CB(from)->tcp_flags;
-	return true;
-}
-
 static void tcp_data_queue_ofo(struct sock *sk, struct sk_buff *skb)
 {
 	struct tcp_sock *tp = tcp_sk(sk);