regression in 4.20.0: physical to VETH (LXC) network bridge (unsure if cgroups is related)

I have since reverted to the working LTS kernel image offered by Arch
Linux (4.19.13), but am willing to re-test / gather data additional
data on a couple lower-use time periods during the week.


After updating to Linux 4.20.0 (along with a full system update
otherwise) my BRIDGED network connections to some LXC containers
ceased working.

Attempting to troubleshoot this issue also produced extremely odd
results, which I think offhand MIGHT have caused network packets to
fill up some kind of memory buffer instead of being relaid or dropped;
there are some additional details at the serverfault and LXC bugs that
I filed, as it was initially (and still is) unclear where the actual
issue is.

-

At this time I am unsure if it is related to netdev (bridge, veth),
cgroups, or some changed default that should now be configured in a
way that is different to previous defaults.

https://serverfault.com/questions/947848/linux-bridge-broken-after-upgrade-out-of-ideas-places-to-look-now-4-20-0-arc

https://github.com/lxc/lxc/issues/2769


Also now filed: https://bugzilla.kernel.org/show_bug.cgi?id=202235

* It is NOT related to IP forwarding. This is a BRIDGED connection,
not a routed one, and it works on older kernels without that enabled.

* physical network to bridge works (and will stay connected for a few
min after later troubleshooting steps, even if ARP caches / ping flake
out and stop responding)

* VETH (within LXC) can ping the the host IP on the bridge (but not
the gateway, the host can before this step) if manually assigned a
static address. Doing this seems to cause general instability and a
timed out SSH session. This lead me to reboot between each round of
testing to ensure I had a clean slate to start with.

I went over the major settings that I did check in the other two bug
reports, but I'm open to checking other values and/or performing
different kinds of tests occasionally over a given week. Responses
won't be immediate but I'll try to check on this frequently over the
next two weeks.