[ovs-dev] ovs-vswitchd is resetting the MTU of a bridge when a patch port is deleted.

[Numan Siddique]

Hello,

Daniel (CC'd) and I were debugging an issue in openstack tripleo CI for OVN
job.
We are noticing an issue when ovn-controller deletes the patch ports in the
external bridge specified in "ovn-bridge-mappings". The external bridge
(br-ex) is configured with MTU 1450 as it has an vxlan port. (TripleO CI
 setup configures the MTU).

When ovn-controller deletes the patch port in br-ex, ovs-vswitchd is
changing the MTU of br-ex to 1500 and this is causing problem.

We are able to reproduce the issue by the below commands. This issue is
seen only once. If we re-add the patch ports to br1, br2 we don't see the
issue. We can reproduce the issue either if we delete the bridges and
recreate again or restart ovs-vswitchd.

***********
ovs-vsctl add-br br1
sudo ip link set br1 mtu 1450
ovs-vsctl add-port br1 br1-p1
ovs-vsctl set Interface br1-p1 type=patch
ovs-vsctl set Interface br1-p1 options:peer=br2-p1

sleep 1

ovs-vsctl add-br br2
sudo ip link set br2 mtu 1450
ovs-vsctl add-port br2 br2-p1
ovs-vsctl set Interface br2-p1 type=patch
ovs-vsctl set Interface br2-p1 options:peer=br1-p1

ip a
ovs-vsctl show
ovs-vsctl del-port br1-p1
ip a s br1
************

The below patch fixes the issue.
I am not very sure if this is the right fix. I will submit the patch anyway.

If someone has  a better fix please override the patch.

---
 ofproto/ofproto.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/ofproto/ofproto.c b/ofproto/ofproto.c
index 7541af0b2..9950897b8 100644
--- a/ofproto/ofproto.c
+++ b/ofproto/ofproto.c
@@ -2721,18 +2721,20 @@ init_ports(struct ofproto *p)
 }

 static bool
-ofport_is_internal(const struct ofproto *p, const struct ofport *port)
+ofport_is_internal_or_patch(const struct ofproto *p, const struct ofport
*port)
 {
     return !strcmp(netdev_get_type(port->netdev),
-                   ofproto_port_open_type(p->type, "internal"));
+                   ofproto_port_open_type(p->type, "internal")) ||
+           !strcmp(netdev_get_type(port->netdev),
+                   ofproto_port_open_type(p->type, "patch"));
 }

-/* If 'port' is internal and if the user didn't explicitly specify an mtu
- * through the database, we have to override it. */
+/* If 'port' is internal or patch and if the user didn't explicitly
specify an
+ * mtu through the database, we have to override it. */
 static bool
 ofport_is_mtu_overridden(const struct ofproto *p, const struct ofport
*port)
 {
-    return ofport_is_internal(p, port)
+    return ofport_is_internal_or_patch(p, port)
            && !netdev_mtu_is_user_config(port->netdev);
 }

--