[PATCH net-next v2 1/4] tcp: initialize MTU probing before syncing MSS at connection setup

[Yongjian Wang via B4 Relay]

From: Yongjian Wang <echoowall@xxxxxxxxx>

With tcp_mtu_probing=2, a passive connection starts at the full MSS
rather than tcp_base_mss. The IPv4 and IPv6 syn_recv_sock() paths
call tcp_sync_mss() before tcp_init_transfer() enables MTU probing.
With tcp_base_mss=1024, the first successful probe can therefore lower
the MSS from 1460 to 1242. On a black hole path, the first reduction
skips 1024 and goes straight to 512.

Commit 4d83e1773031 ("tcp: delete redundant calls of tcp_mtup_init()")
removed tcp_mtup_init() from the passive setup paths.
Commit 27204aaa9dc6
("tcp: uniform the set up of sockets after successful connection")
later moved MTU probing initialization into tcp_init_transfer(),
away from tcp_sync_mss().

Initialize MTU probing immediately before the existing tcp_sync_mss()
calls in the IPv4 and IPv6 passive setup paths and when processing
a SYN-ACK for an active connection. Remove the late initialization
from tcp_init_transfer(). This lets tcp_sync_mss() apply tcp_base_mss
without adding another MSS calculation to connection setup. The
crossed-SYN path already uses this order.

Fixes: 4d83e1773031 ("tcp: delete redundant calls of tcp_mtup_init()")
Suggested-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>
Suggested-by: Neal Cardwell <ncardwell@xxxxxxxxxx>
Signed-off-by: Yongjian Wang <echoowall@xxxxxxxxx>
---
 net/ipv4/tcp_input.c | 2 +-
 net/ipv4/tcp_ipv4.c  | 1 +
 net/ipv6/tcp_ipv6.c  | 1 +
 3 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 3fd0e2153..2b54b99f1 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -6763,7 +6763,6 @@ void tcp_init_transfer(struct sock *sk, int bpf_op, struct sk_buff *skb)
 	struct inet_connection_sock *icsk = inet_csk(sk);
 	struct tcp_sock *tp = tcp_sk(sk);
 
-	tcp_mtup_init(sk);
 	icsk->icsk_af_ops->rebuild_header(sk);
 	tcp_init_metrics(sk);
 
@@ -7020,6 +7019,7 @@ static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
 			tp->tcp_header_len = sizeof(struct tcphdr);
 		}
 
+		tcp_mtup_init(sk);
 		tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
 		tcp_initialize_rcv_mss(sk);
 
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 0dc3b0876..fb968798b 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1740,6 +1740,7 @@ struct sock *tcp_v4_syn_recv_sock(const struct sock *sk, struct sk_buff *skb,
 
 	tcp_ca_openreq_child(newsk, dst);
 
+	tcp_mtup_init(newsk);
 	tcp_sync_mss(newsk, dst4_mtu(dst));
 	newtp->advmss = tcp_mss_clamp(tcp_sk(sk), tcp_dst_advmss(dst));
 
diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index b1dec5d86..8ea1ae1ad 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -1490,6 +1490,7 @@ static struct sock *tcp_v6_syn_recv_sock(const struct sock *sk, struct sk_buff *
 
 	tcp_ca_openreq_child(newsk, dst);
 
+	tcp_mtup_init(newsk);
 	tcp_sync_mss(newsk, dst6_mtu(dst));
 	newtp->advmss = tcp_mss_clamp(tcp_sk(sk), tcp_dst_advmss(dst));
 

-- 
Git-157)