[RFC PATCH v4 net-next 2/4] 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 <redacted>
---
 include/net/tcp.h | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 1e6c5b04..7c510ed 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h

@@ -755,11 +755,16 @@ struct tcp_skb_cb {
 	/* 1 byte hole */
 	__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]))

-- 
1.8.1