Re: [PATCH 0/6] Assorted patches relating to mdmon

[Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>]

On Mon, 27 Feb 2023 11:13:07 +1100
NeilBrown <neilb@suse.de> wrote:

> 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
> 

Hi Jes,
The problem descried by Neil is critical for IMSM. I will test the patchset
ASAP.
Additionally, it resolves "KillMode=none" problem so we will be able to finally
drop it.

I will be back with results soon.

Thanks,
Mariusz