While debugging some OVN issues, sometimes it's handy to create a namespace
and a port inside it (this is quicker than create VM's and we don't need OpenStack).
So while doing this on one of the compute nodes, we saw an error
with the OVN metadata agent where it was unable to set the ovnmeta namespace.

We have been digging this morning and this is not apparently an OVN related
issue. It's easily reproducible with the following steps:

1. Create a namespace within a container:

# docker run --net=host --user=root --privileged=True  -v /run/netns:/run/netns:shared -it  f45d92a693e8 /bin/bash
()[root@overcloud-controller-0 /]# ip netns add netns1

2. Create a namespace in the host

[root@overcloud-controller-0 heat-admin]# ip netns add netns2
[root@overcloud-controller-0 heat-admin]# ip netns
RTNETLINK answers: Invalid argument
netns2
RTNETLINK answers: Invalid argument
netns1

3. The namespaces become unusable. From the container:


()[root@overcloud-controller-0 /]# ip netns
RTNETLINK answers: Invalid argument
netns2
RTNETLINK answers: Invalid argument
netns1
()[root@overcloud-controller-0 /]# ip netns e netns1 ip a
RTNETLINK answers: Invalid argument
setting the network namespace "netns1" failed: Invalid argument


Only way to recover from this is rebooting the node. So I don't know how unlikely is that
someone creates a network namespace in a controller but if that's done, basically
all the existing namespaces (qrouter, qdhcp) become unusable and dataplane would be disrupted.

We have tried in both RHEL 7.5 and CentOS 7.4. 
CentOS kernel is: Linux overcloud-controller-0 3.10.0-693.21.1.el7.x86_64 #1 SMP Wed Mar 7 19:03:37 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

As this was a TripleO virt setup, we wanted to confirm and try the same on the
baremetal node. Spawning a container there mounting the /run dir as shared
didn't work so we changed the following file:

/etc/systemd/system/docker.service
- MountFlags=slave
+MountFlags=shared

Restarted docker and then we could create namespaces both inside and outside
the containers.