tcp: refactor struct tcp_skb_cb

Refactor tcp_skb_cb to create two overlaping areas to store
state for incoming or outgoing skbs based on comments by
Neal Cardwell to tcp_nv patch:

   AFAICT this patch would not require an increase in the size of
   sk_buff cb[] if it were to take advantage of the fact that the
   tcp_skb_cb header.h4 and header.h6 fields are only used in the packet
   reception code path, and this in_flight field is only used on the
   transmit side.

Signed-off-by: Lawrence Brakmo <brakmo@fb.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 8 insertions, 3 deletions

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 24ec80483805..4775a1bba7f7 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -765,11 +765,16 @@ struct tcp_skb_cb {
                         unused:6;
         __u32           ack_seq;        /* Sequence number ACK'd        */
         union {
-                struct inet_skb_parm    h4;
+                struct {
+                        /* There is space for up to 20 bytes */
+                } tx;   /* only used for outgoing skbs */
+                union {
+                        struct inet_skb_parm    h4;
 #if IS_ENABLED(CONFIG_IPV6)
-                struct inet6_skb_parm   h6;
+                        struct inet6_skb_parm   h6;
 #endif
-        } header;       /* For incoming frames          */
+                } header;       /* For incoming skbs */
+        };
 };
 
 #define TCP_SKB_CB(__skb)       ((struct tcp_skb_cb *)&((__skb)->cb[0]))