netdev - [RFC PATCH v5 net-next 2/4] tcp: refactor struct tcp_skb

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Date:	Tue, 4 Aug 2015 18:39:06 -0700
From:	Lawrence Brakmo <brakmo@...com>
To:	netdev <netdev@...r.kernel.org>
CC:	Kernel Team <kernel-team@...com>,
	Neal Cardwell <ncardwell@...gle.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Yuchung Cheng <ycheng@...gle.com>,
	Stephen Hemminger <stephen@...workplumber.org>
Subject: [RFC PATCH v5 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 <brakmo@...com>
---
 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 0121529..a086a98 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]))
-- 
1.8.5.6

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