Comment 2 for bug 1876018

> Apr 29 22:36:46 focal01 systemd-udevd[266]: memory7: /usr/lib/udev/rules.d/40-vm-hotadd.rules:9 Failed to write ATTR{/sys/devices/system/memory/memory7/state}, ignoring: Invalid argument

This isn't because the file doesn't exist, because the udev rule checks for its existence before trying to write to it. The error is because the kernel is rejecting the write to its state file for some reason. In a really quick test, it seems like this is because it's already online; the kernel default has been CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE=y since at least 4.15.

I think the error message is harmless, since the memory is already online, but the udev rule should be fixed to check if it's offline before actually trying to online it.

Can you test with the systemd from this ppa to see if the error message is gone?
https://launchpad.net/~ddstreet/+archive/ubuntu/lp1876018

Also please make sure all your memory and cpus are online after booting/hotadding.

Note that I might just remove this rule entirely, since it really shouldn't be needed anymore, now that the kernel default is to online hotplugged cpus/memory, but I want to verify that first.