[PATCH net-next 1/2] tcp: record last received ipv6 flowlabel

[David Morley <morleyd.kernel@gmail.com>]

From: David Morley <morleyd@google.com>

In order to better estimate whether a data packet has been
retransmitted or is the result of a TLP, we save the last received
ipv6 flowlabel.

To make space for this field we resize the "ato" field in
inet_connection_sock as the current value of TCP_DELACK_MAX can be
fully contained in 8 bits and add a compile_time_assert ensuring this
field is the required size.

Signed-off-by: David Morley <morleyd@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Tested-by: David Morley <morleyd@google.com>
---
 include/net/inet_connection_sock.h |  5 ++++-
 include/net/tcp.h                  |  2 ++
 net/ipv4/tcp_input.c               | 15 +++++++++++++++
 net/ipv4/tcp_timer.c               |  2 +-
 4 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/include/net/inet_connection_sock.h b/include/net/inet_connection_sock.h
index 5d2fcc137b88..d6d9d1c1985a 100644
--- a/include/net/inet_connection_sock.h
+++ b/include/net/inet_connection_sock.h
@@ -114,7 +114,10 @@ struct inet_connection_sock {
 		__u8		  quick;	 /* Scheduled number of quick acks	   */
 		__u8		  pingpong;	 /* The session is interactive		   */
 		__u8		  retry;	 /* Number of attempts			   */
-		__u32		  ato;		 /* Predicted tick of soft clock	   */
+		#define ATO_BITS 8
+		__u32		  ato:ATO_BITS,	 /* Predicted tick of soft clock	   */
+				  lrcv_flowlabel:20, /* last received ipv6 flowlabel	   */
+				  unused:4;
 		unsigned long	  timeout;	 /* Currently scheduled timeout		   */
 		__u32		  lrcvtime;	 /* timestamp of last received data packet */
 		__u16		  last_seg_size; /* Size of last incoming segment	   */
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 91688d0dadcd..8a3720c7d082 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -131,6 +131,8 @@ void tcp_time_wait(struct sock *sk, int state, int timeo);
 #define TCP_FIN_TIMEOUT_MAX (120 * HZ) /* max TCP_LINGER2 value (two minutes) */
 
 #define TCP_DELACK_MAX	((unsigned)(HZ/5))	/* maximal time to delay before sending an ACK */
+static_assert(1<<ATO_BITS > TCP_DELACK_MAX);
+
 #if HZ >= 100
 #define TCP_DELACK_MIN	((unsigned)(HZ/25))	/* minimal time to delay before sending an ACK */
 #define TCP_ATO_MIN	((unsigned)(HZ/25))
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 584825ddd0a0..abe7494361c0 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -765,6 +765,16 @@ void tcp_rcv_space_adjust(struct sock *sk)
 	tp->rcvq_space.time = tp->tcp_mstamp;
 }
 
+static void tcp_save_lrcv_flowlabel(struct sock *sk, const struct sk_buff *skb)
+{
+#if IS_ENABLED(CONFIG_IPV6)
+	struct inet_connection_sock *icsk = inet_csk(sk);
+
+	if (skb->protocol == htons(ETH_P_IPV6))
+		icsk->icsk_ack.lrcv_flowlabel = ntohl(ip6_flowlabel(ipv6_hdr(skb)));
+#endif
+}
+
 /* There is something which you must keep in mind when you analyze the
  * behavior of the tp->ato delayed ack timeout interval.  When a
  * connection starts up, we want to ack as quickly as possible.  The
@@ -813,6 +823,7 @@ static void tcp_event_data_recv(struct sock *sk, struct sk_buff *skb)
 		}
 	}
 	icsk->icsk_ack.lrcvtime = now;
+	tcp_save_lrcv_flowlabel(sk, skb);
 
 	tcp_ecn_check_ce(sk, skb);
 
@@ -4506,6 +4517,9 @@ static void tcp_rcv_spurious_retrans(struct sock *sk, const struct sk_buff *skb)
 	if (TCP_SKB_CB(skb)->seq == tcp_sk(sk)->duplicate_sack[0].start_seq &&
 	    sk_rethink_txhash(sk))
 		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDUPLICATEDATAREHASH);
+
+	/* Save last flowlabel after a spurious retrans. */
+	tcp_save_lrcv_flowlabel(sk, skb);
 }
 
 static void tcp_send_dupack(struct sock *sk, const struct sk_buff *skb)
@@ -4822,6 +4836,7 @@ static void tcp_data_queue_ofo(struct sock *sk, struct sk_buff *skb)
 	u32 seq, end_seq;
 	bool fragstolen;
 
+	tcp_save_lrcv_flowlabel(sk, skb);
 	tcp_ecn_check_ce(sk, skb);
 
 	if (unlikely(tcp_try_rmem_schedule(sk, skb, skb->truesize))) {
diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index 3f61c6a70a1f..0862b73dd3b5 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -322,7 +322,7 @@ void tcp_delack_timer_handler(struct sock *sk)
 	if (inet_csk_ack_scheduled(sk)) {
 		if (!inet_csk_in_pingpong_mode(sk)) {
 			/* Delayed ACK missed: inflate ATO. */
-			icsk->icsk_ack.ato = min(icsk->icsk_ack.ato << 1, icsk->icsk_rto);
+			icsk->icsk_ack.ato = min_t(u32, icsk->icsk_ack.ato << 1, icsk->icsk_rto);
 		} else {
 			/* Delayed ACK missed: leave pingpong mode and
 			 * deflate ATO.
-- 
2.42.0.582.g8ccd20d70d-goog