Comment # 18 on bug 1214845 from Eric van Blokland

Ok now I have better understanding of the problem... 

In my opinion DefaultMemoryAccounting=Yes is not and should not be required for
lxc containers to run.

Some change, presumable in systemd or its configuration, made this issue
surface. At a first glance I think this is an lxc/systemd interoperability
issue. 

If I read the source correctly, the libvirt LXC driver assumes it can check
cgroup controller availability with its own cgroup. Under the current
circumstances this assumption is wrong.

When the LXC driver is run with systemd, the container process cgroup is moved
under machine.slice, where the memory controller is available regardless of the
DefaultMemoryAccounting setting.

If the current behaviour/configuration of systemd/cgroups is correct and
desired I could write a "crude" fix for the LXC driver to not check controller
availability when started with systemd and the cgroupv2 backend.