[PATCH net v2 1/2] tcp_metrics: set congestion window clamp from the dst entry

[Petr Tesarik <ptesarik@suse.com>]

If RTAX_CWND is locked, always initialize tp->snd_cwnd_clamp from the
corresponding dst entry.

Note that an unlocked RTAX_CWND does not have any effect on the kernel.
This behavior is even documented in the manual page of ip-route(8):

    cwnd NUMBER (Linux 2.3.15+ only)
	   the clamp for congestion window. It is ignored if the lock
	   flag is not used.

An unlocked RTAX_CWND was updated by tcp_update_metrics() until v3.6. Since
then, only the newly introduced TCP metric (TCP_METRIC_CWND) has been
updated, rendering unlocked RTAX_CWND useless.

TCP metrics are updated after a TCP connection finishes. If there are no
metrics for a given destination when a new connection is created, default
values are used instead.

This means there are two issues with setting tp->snd_cwnd_clamp from the
TCP metric:

1. If the cwnd option is changed in the routing table, the new value is not
   used for new connections as long as there is a cached TCP metric for the
   destination. An existing cached metric is not updated from the routing
   table unless it has seen no update for longer than TCP_METRICS_TIMEOUT
   (1 hour).

2. After evicting the corresponding cached metric, the new value from the
   routing table is still not used for new connections until one connection
   finishes, and a new cached entry is created.

As a result, the following shenanigan is required to set a new locked cwnd
clamp:

- update the route (``ip route replace ... cwnd lock $value``)
- flush any existing TCP metric entry (``ip tcp_metrics flush $dest``)
- create and finish a dummy connection to the destination to create a TCP
  metric entry with the new value
- *next* connection to this destination will use the new value

The above does not seem to be intentional.

NB there is also an initcwnd route parameter (RTAX_INITCWND) to set the
initial size of the congestion window; this patch does not change anything
about the handling of that parameter.

Fixes: 51c5d0c4b169 ("tcp: Maintain dynamic metrics in local cache.")
Signed-off-by: Petr Tesarik <ptesarik@suse.com>
---
 net/ipv4/tcp_metrics.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/tcp_metrics.c b/net/ipv4/tcp_metrics.c
index 4251670e328c..dd8f3457bd72 100644
--- a/net/ipv4/tcp_metrics.c
+++ b/net/ipv4/tcp_metrics.c
@@ -477,6 +477,9 @@ void tcp_init_metrics(struct sock *sk)
 	if (!dst)
 		goto reset;
 
+	if (dst_metric_locked(dst, RTAX_CWND))
+		tp->snd_cwnd_clamp = dst_metric(dst, RTAX_CWND);
+
 	rcu_read_lock();
 	tm = tcp_get_metrics(sk, dst, false);
 	if (!tm) {
@@ -484,9 +487,6 @@ void tcp_init_metrics(struct sock *sk)
 		goto reset;
 	}
 
-	if (tcp_metric_locked(tm, TCP_METRIC_CWND))
-		tp->snd_cwnd_clamp = tcp_metric_get(tm, TCP_METRIC_CWND);
-
 	val = READ_ONCE(net->ipv4.sysctl_tcp_no_ssthresh_metrics_save) ?
 	      0 : tcp_metric_get(tm, TCP_METRIC_SSTHRESH);
 	if (val) {
-- 
2.49.0