On 3/12/23 23:42, NeilBrown wrote:
> This series is a minor revision of the previous series with the same
> name.
> It includes the important bugfix identified by Mariusz and a few minor
> improvements suggested by Paul.
>
> Original comment:
>
> mdmon is a root-storage daemon is the sense defined by systemd
> documentation, but it does not follow the practice that systemd
> recommends. Specifically it is run from the root filesystem when
> possible. The instance started in the initrd hands-over to a root-fs
> based instance, which then hands-over to an initrd-based instance
> started by dracut at shutdown.
>
> Part of the reason that we ignore systemd advise is that mdmon needs
> access to the filesystem - specifically /dev and /sys - which is not
> available in the initrd context after switchroot. We could possibly
> change mdmon to work in the systemd-preferred way by splitting mdmon
> into two processes instead of just having 2 threads. The "monitor"
> process could running entirely in the initrd context, the "manager"
> process could safely run in the root-fs context, passing newly opened
> file descriptors to the monitor over a unix-domain socket.
>
> But we aren't there yet and may never be.
>
> For now, mdmon doesn't work correctly. There is no mechanism to ensure
> a new instance starts after switchroot. Until recently the initrd
> instance of the systemd mdmon unit would be stopped at switchroot time
> because udev would temporarily forget about md devices. This would
> allow the "udevadm trigger" process to start a new instance. udev was
> recently fixed:
>
> Commit: 7ec624147a41 ("udevadm: cleanup-db: don't delete information for kept db entries")
>
> so now the attempt to start mdmon via "udevadm trigger" does nothing as
> mdmon already has an active unit.
>
> The net result is that mdmon continues running in the initrd mount
> namespace and so cannot access new devices. Adding a device to a root
> md array that depends on mdmon will no longer work.
>
> We want the initrd instance of mdmon to continue running until the
> root-fs based instance starts, and that really requires we have two
> different systemd units. This series achieves this in the final patch by
> using a different instance name inside or initrd and outside.
> "initrd-mdfoo" and "mdfoo".
>
> Other patches in the series are mostly clean-ups and minor improvements
> in related code.
>
> NeilBrown
Applied, 1, 3-6. Skipping 2 per previous email.
Thanks,
Jes
