[TCP]: Adjust Reno SACK estimate in tcp_fragment

Since the introduction of TSO pcount a year ago, it has been possible
for tcp_fragment() to cause packets_out to decrease.  Prior to that,
tcp_retrans_try_collapse() was the only way for that to happen on the
retransmission path.

When this happens with Reno, it is possible for sasked_out to become
invalid because it is only an estimate and not tied to any particular
packet on the retransmission queue.

Therefore we need to adjust sacked_out as well as left_out in the Reno
case.  The following patch does exactly that.

This bug is pretty difficult to trigger in practice though since you
need a SACKless peer with a retransmission that occurs just as the
cached MTU value expires.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 9 insertions, 0 deletions

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 5dd6dd7d091e..d6e3d269e906 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -509,7 +509,16 @@ int tcp_fragment(struct sock *sk, struct sk_buff *skb, u32 len, unsigned int mss
                         tp->lost_out -= diff;
                         tp->left_out -= diff;
                 }
+
                 if (diff > 0) {
+                        /* Adjust Reno SACK estimate. */
+                        if (!tp->rx_opt.sack_ok) {
+                                tp->sacked_out -= diff;
+                                if ((int)tp->sacked_out < 0)
+                                        tp->sacked_out = 0;
+                                tcp_sync_left_out(tp);
+                        }
+
                         tp->fackets_out -= diff;
                         if ((int)tp->fackets_out < 0)
                                 tp->fackets_out = 0;