[TCP]: Consider GSO while counting reord in sacktag

Reordering detection fails to take account that the reordered
skb may have pcount larger than 1. In such case the lowest of
them had the largest reordering, the old formula used the
highest of them which is pcount - 1 packets less reordered.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index ca9590f..0f75757 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1403,8 +1403,6 @@
 			if (in_sack < 0)
 				break;
 
-			fack_count += tcp_skb_pcount(skb);
-
 			sacked = TCP_SKB_CB(skb)->sacked;
 
 			/* Account D-SACK for retransmitted packet. */
@@ -1427,11 +1425,14 @@
 				}
 
 				/* Nothing to do; acked frame is about to be dropped. */
+				fack_count += tcp_skb_pcount(skb);
 				continue;
 			}
 
-			if (!in_sack)
+			if (!in_sack) {
+				fack_count += tcp_skb_pcount(skb);
 				continue;
+			}
 
 			if (!(sacked&TCPCB_SACKED_ACKED)) {
 				if (sacked & TCPCB_SACKED_RETRANS) {
@@ -1480,6 +1481,7 @@
 				flag |= FLAG_DATA_SACKED;
 				tp->sacked_out += tcp_skb_pcount(skb);
 
+				fack_count += tcp_skb_pcount(skb);
 				if (fack_count > tp->fackets_out)
 					tp->fackets_out = fack_count;
 
@@ -1490,6 +1492,8 @@
 			} else {
 				if (dup_sack && (sacked&TCPCB_RETRANS))
 					reord = min(fack_count, reord);
+
+				fack_count += tcp_skb_pcount(skb);
 			}
 
 			/* D-SACK. We can detect redundant retransmission
@@ -1515,7 +1519,7 @@
 
 	if ((reord < tp->fackets_out) && icsk->icsk_ca_state != TCP_CA_Loss &&
 	    (!tp->frto_highmark || after(tp->snd_una, tp->frto_highmark)))
-		tcp_update_reordering(sk, ((tp->fackets_out + 1) - reord), 0);
+		tcp_update_reordering(sk, tp->fackets_out - reord, 0);
 
 #if FASTRETRANS_DEBUG > 0
 	BUG_TRAP((int)tp->sacked_out >= 0);