nixos/ksm: rewrite using systemd-tmpfiles

[rnhmjoj]

Motivation for this change

This is to get rid of the ugly systemd-udev-settle dependency.
See #73095 for more information.

Things done

• Tested via one or more NixOS test (nixosTests.ksm)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[flokli]

How would moving this to tmpfiles fix the problem? This seems to be a script writing to /sys, possibly requiring these files to be present (and underlying hardware to be appropriately initialized), hence the dependency on udevadm-settle.

Nothing guarantees systemd-tmpfiles runs "late enough" for this to has happened.

Can this be some udev rule instead?

[flokli]

RedHat seems to have some daemon here, provided by qemu: https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/6/html/virtualization_tuning_and_optimization_guide/chap-ksm

[rnhmjoj]

How would moving this to tmpfiles fix the problem?

It changes nothing in this regard, but I don't think there is a problem to begin with, at least not anymore. I don't see why /sys/kernel/mm/ksm/ shouldn't be available as soon as the kernel is executed.

Try building a system with:

boot.initrd.preDeviceCommands = ''
 echo 1 > /sys/kernel/mm/ksm/run; cat /sys/kernel/mm/ksm/run
'';

this runs in stage 1, before udev is even started and succeeds.

Btw, I stole the tmpfiles rule from https://wiki.archlinux.org/index.php/QEMU#Enabling_KSM.

[flokli]

In that case, I'd prefer if this could stay a shell script, and we just drop the dependency. If we do that, worst case the script will fail (and people will realize). If this is switched to a tmpfiles rule, it might do the right thing every second time, and every other time it'll try to create the underlying file structure by itself ;-)

[rnhmjoj]

and every other time it'll try to create the underlying file structure by itself ;-)

According to tmpfiles.d(5) this should never happen: if the file does not exist it should simply fail, but ok I'll keep the script.

[flokli]

don't usepkgs.lib, but lib directly