Re: [systemd-devel] systemd kills mdmon if it was started manually by user

[NeilBrown <neilb@suse.de>]

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On Wed, 2 Nov 2011 14:32:25 +0100 Lennart Poettering <lennart@poettering.net>
wrote:

> On Wed, 02.11.11 13:03, NeilBrown (neilb@suse.de) wrote:

> > Each instance of mdmon manages a set of arrays and must remain running
> > until all of those arrays are readonly (or shut down).  This allows it to
> > record that all writes have completed and mark the array as 'clean' so a
> > resync isn't needed at next boot.
> 
> Why doesn't the kernel do that on its own?

Because the kernel doesn't know about the format of the metadata that
describes the array.

> > 
> > You couldn't just do the equivalent of
> >   fuser -k /some/filesystem
> >   umount /some/filesystem
> > 
> > iterating over filesystems with '/' last?
> >
> > Then anything that only uses the /run filesystem will survive.
> 
> What we do right now is this:
> 
> kill_all_processes();
> do {
>      umount_all_file_systems_we_can();
>      read_only_mount_all_remaining_file_systems();
> } while (we_had_some_success_with_that());
> jump_into_initrd();
> 
> As long as mdmon references a file from the root disk we cannot umount
> it, so the loop wouldn't be effective.

What exactly is "kill_all_processes()"?   is it SIGTERM or SIGKILL or both
with a gap or ???

I assume a SIGKILL.  I don't mind a SIGTERM and it could be useful to
expedite mdmon cleaning up.

However there is an important piece missing.  When you remount,ro a
filesystem, the block device doesn't get told so it thinks it is still open
read/write.  So md cannot tell mdmon that the array is now read-only
It would make a lot of sense for mdmon to exit after receiving a SIGTERM as
soon as the device is marked read-only.  But it just doesn't know.

We can probably fix that, but that doesn't really help for now.

I think I would like:

 - add to the above loop "stop any virtual devices that we can".
   Exactly how to do that if /proc and /sys are already unmounted
   is unclear.  Is one or both of these kept around somewhere?

 - allow processes to be marked some way so they get SIGTERM but not
   SIGKILL.  I'm happy adding magic char to argv[0].

We should be able to make it work with those changes - if they are possible.

Thanks,

NeilBrown


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