[TCP]: Make fackets_out accurate

Substraction for fackets_out is unconditional when snd_una
advances, thus there's no need to do it inside the loop. Just
make sure correct bounds are honored.

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

1 files changed, 26 insertions, 18 deletions

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 9df5b2a4da6e..cbe8bf6dab5f 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -652,6 +652,26 @@ static void tcp_set_skb_tso_segs(struct sock *sk, struct sk_buff *skb, unsigned
         }
 }
 
+/* When a modification to fackets out becomes necessary, we need to check
+ * skb is counted to fackets_out or not. Another important thing is to
+ * tweak SACK fastpath hint too as it would overwrite all changes unless
+ * hint is also changed.
+ */
+static void tcp_adjust_fackets_out(struct tcp_sock *tp, struct sk_buff *skb,
+                                   int decr)
+{
+        if (!tp->sacked_out)
+                return;
+
+        if (!before(tp->highest_sack, TCP_SKB_CB(skb)->seq))
+                tp->fackets_out -= decr;
+
+        /* cnt_hint is "off-by-one" compared with fackets_out (see sacktag) */
+        if (tp->fastpath_skb_hint != NULL &&
+            after(TCP_SKB_CB(tp->fastpath_skb_hint)->seq, TCP_SKB_CB(skb)->seq))
+                tp->fastpath_cnt_hint -= decr;
+}
+
 /* Function to create two new TCP segments.  Shrinks the given segment
  * to the specified size and appends a new segment with the rest of the
  * packet to the list.  This won't be called frequently, I hope.
@@ -746,21 +766,12 @@ int tcp_fragment(struct sock *sk, struct sk_buff *skb, u32 len, unsigned int mss
                 if (TCP_SKB_CB(skb)->sacked & TCPCB_LOST)
                         tp->lost_out -= diff;
 
-                if (diff > 0) {
-                        /* Adjust Reno SACK estimate. */
-                        if (tcp_is_reno(tp)) {
-                                tcp_dec_pcount_approx_int(&tp->sacked_out, diff);
-                                tcp_verify_left_out(tp);
-                        }
-
-                        tcp_dec_pcount_approx_int(&tp->fackets_out, diff);
-                        /* SACK fastpath might overwrite it unless dealt with */
-                        if (tp->fastpath_skb_hint != NULL &&
-                            after(TCP_SKB_CB(tp->fastpath_skb_hint)->seq,
-                                  TCP_SKB_CB(skb)->seq)) {
-                                tcp_dec_pcount_approx_int(&tp->fastpath_cnt_hint, diff);
-                        }
+                /* Adjust Reno SACK estimate. */
+                if (tcp_is_reno(tp) && diff > 0) {
+                        tcp_dec_pcount_approx_int(&tp->sacked_out, diff);
+                        tcp_verify_left_out(tp);
                 }
+                tcp_adjust_fackets_out(tp, skb, diff);
         }
 
         /* Link BUFF into the send queue. */
@@ -1746,10 +1757,7 @@ static void tcp_retrans_try_collapse(struct sock *sk, struct sk_buff *skb, int m
                 if (tcp_is_reno(tp) && tp->sacked_out)
                         tcp_dec_pcount_approx(&tp->sacked_out, next_skb);
 
-                /* Not quite right: it can be > snd.fack, but
-                 * it is better to underestimate fackets.
-                 */
-                tcp_dec_pcount_approx(&tp->fackets_out, next_skb);
+                tcp_adjust_fackets_out(tp, skb, tcp_skb_pcount(next_skb));
                 tp->packets_out -= tcp_skb_pcount(next_skb);
                 sk_stream_free_skb(sk, next_skb);
         }