Re: why is the same mount point repeatedly mounted in nested manner?

[Dave T <davestechshop@gmail.com>]

On Mon, Aug 9, 2021 at 3:29 PM Goffredo Baroncelli <kreijack@libero.it> wrote:
>
> On 8/8/21 9:48 PM, Dave T wrote:
> > On Sun, Aug 8, 2021 at 8:10 AM <devel@roosoft.ltd.uk> wrote:
> >>
> >> On 05/08/2021 17:46, Dave T wrote:
> [...]
> >>
> >> Try mounting the subvolume with it's subvolume ID. System only generates
> >> unit files from the fstab it does not follow them , so if you are vague
> >> in your fstab the systemd unit files will also be vague.
> >
> > Thank you for the tip. I appreciate your interest in my issue.
> > However, I don't fully understand what to change.
>
> I think that Alexander is suggesting to add something like ',subvolid=5' to the line of fstab where /mnt/btrtop/root is mounted.

I see. Thank you (and Alexander). I will add that! EDIT: I just added
it and tested mounting, umounting. It seems to work exactly the same
as previously. findmnt now reports:

├─/mnt/btrtop/root                       /dev/mapper/rootluks
                               btrfs
rw,noatime,nodiratime,compress=lzo,space_cache,subvolid=5,subvol=/

>
> I add that if it is a systemd bug, it would help to look at the .mount files generated by systemd:
>
> $ sudo ls /run/systemd/generator/*.mount
>
> Can you share the content of the  systemd units where you ask to mount '/mnt/btrtop/root' ? It could help the diagnose.
>
Thank you. Good tip. I had not looked at those files before. Here are
the generator files (before my change to add ,subvolid=5).

# cat /run/systemd/generator/-.mount
# Automatically generated by systemd-fstab-generator
[Unit]
Documentation=man:fstab(5) man:systemd-fstab-generator(8)
SourcePath=/etc/fstab
Before=local-fs.target
After=blockdev@dev-disk-by\x2duuid-28D099A\x2d49D92\x2d4487C\x2d48113\x2d4A231CAD0EEF2.target
[Mount]
Where=/
What=/dev/disk/by-uuid/28D099A-9D92-487C-8113-A231CAD0EEF2
Type=btrfs
Options=rw,noatime,nodiratime,compress=lzo,space_cache,subvol=/@btrtop/snapshot



# cat /run/systemd/generator/mnt-btrtop-root.mount
# Automatically generated by systemd-fstab-generator
[Unit]
Documentation=man:fstab(5) man:systemd-fstab-generator(8)
SourcePath=/etc/fstab
After=blockdev@dev-disk-by\x2duuid-28D099A\x2d49D92\x2d4487C\x2d48113\x2d4A231CAD0EEF2.target
[Mount]
Where=/mnt/btrtop/root
What=/dev/disk/by-uuid/28D099A-9D92-487C-8113-A231CAD0EEF2
Type=btrfs
Options=noauto,nofail,rw,noatime,nodiratime,compress=lzo,space_cache




# cat /run/systemd/generator/srv-nfs-var-cache-pacman.mount
# Automatically generated by systemd-fstab-generator
[Unit]
Documentation=man:fstab(5) man:systemd-fstab-generator(8)
SourcePath=/etc/fstab
Before=local-fs.target
[Mount]
Where=/srv/nfs/var/cache/pacman
What=/var/cache/pacman
Type=none
Options=bind


> And finally, what happens if you mount/unmount from command line (e.g. manually) /mnt/btrtop/root ?

It works as expected. It mounts and unmounts and there are no errors
and the extra nested mounting doesn't occur. I just ran through a
testing cycle of multiple mounts and unmounts to verify this.

Also, in recent days I stopped mounting and umounting /mnt/btrtop/root
and just left it mounted all the time. However, when checking today, I
still found a nested mount:

├─/srv/nfs/var/cache/pacman
│
│ └─/srv/nfs/var/cache/pacman


>
> BR
>
> > Here are the relevant lines from my fstab. I added line numbers
> > because the lines will get wrapped in email.  I don't see what part of
> > this is vague:
> >
> > 1. # cat /etc/fstab
> > 2. UUID=28D099A-9D92-487C-8113-A231CAD0EEF2  /     btrfs
> > rw,noatime,nodiratime,compress=lzo,space_cache,subvol=/@btrtop/snapshot
> > 0 0
> > 3. UUID=28D099A-9D92-487C-8113-A231CAD0EEF2  /mnt/btrtop/root  btrfs
> > noauto,nofail,rw,noatime,nodiratime,compress=lzo,space_cache    0 0
> > 4. /var/cache/pacman       /srv/nfs/var/cache/pacman       none  bind  0 0
>
> I don't know if it matters, but why you set as 'none' the filesystem type ? However according to askubuntu/stackoverflow it seem the right thing to do...

That is the way I have always done bind mounts.

I can send more complete systemd logs if needed, but this is what it
looked like hourly (before I stopped unmounting):

Aug 03 15:00:12 systemd[1]: mnt-btrtop-root.mount: Deactivated successfully.
Aug 03 16:00:06 btrbk_run.sh[2376252]: mounting btrbk btrtop volumes...
Aug 03 16:00:06 btrbk_run.sh[2376252]: OK: mounted [/mnt/btrtop/root] (1 of 3)
Aug 03 16:00:06 btrbk_run.sh[2376252]: OK: mounted [/mnt/btrtop/home] (2 of 3)
Aug 03 16:00:06 btrbk_run.sh[2376252]: OK: mounted mount
[/mnt/btrtop/user] (3 of 3)
Aug 03 16:00:06 btrbk_run.sh[2376252]: Finished mounting btrbk btrtop volumes.
Aug 03 16:00:11 btrbk_run.sh[2376356]: un-mounting btrbk btrtop volumes...
Aug 03 16:00:11 btrbk_run.sh[2376356]: OK: un-mounted
[/mnt/btrtop/user] (1 of 3)
Aug 03 16:00:11 systemd[1]: mnt-btrtop-user.mount: Deactivated successfully.
Aug 03 16:00:11 btrbk_run.sh[2376356]: OK: un-mounted
[/mnt/btrtop/home] (2 of 3)
Aug 03 16:00:11 systemd[1]: mnt-btrtop-home.mount: Deactivated successfully.
Aug 03 16:00:11 btrbk_run.sh[2376356]: OK: un-mounted
[/mnt/btrtop/root] (3 of 3)
Aug 03 16:00:11 btrbk_run.sh[2376356]: Finished un-mounting btrbk
btrtop volumes.
Aug 03 16:00:11 systemd[1]: mnt-btrtop-root.mount: Deactivated successfully.
Aug 03 17:00:06 btrbk_run.sh[2387185]: mounting btrbk btrtop volumes...
Aug 03 17:00:06 btrbk_run.sh[2387185]: OK: mounted [/mnt/btrtop/root] (1 of 3)
Aug 03 17:00:06 btrbk_run.sh[2387185]: OK: mounted [/mnt/btrtop/home] (2 of 3)
Aug 03 17:00:06 btrbk_run.sh[2387185]: OK: mounted mount
[/mnt/btrtop/user] (3 of 3)
Aug 03 17:00:06 btrbk_run.sh[2387185]: Finished mounting btrbk btrtop volumes.
Aug 03 17:00:10 btrbk_run.sh[2387275]: un-mounting btrbk btrtop volumes...
Aug 03 17:00:10 btrbk_run.sh[2387275]: OK: un-mounted
[/mnt/btrtop/user] (1 of 3)
Aug 03 17:00:10 systemd[1]: mnt-btrtop-user.mount: Deactivated successfully.
Aug 03 17:00:10 btrbk_run.sh[2387275]: OK: un-mounted
[/mnt/btrtop/home] (2 of 3)
Aug 03 17:00:10 systemd[1]: mnt-btrtop-home.mount: Deactivated successfully.
Aug 03 17:00:10 btrbk_run.sh[2387275]: OK: un-mounted
[/mnt/btrtop/root] (3 of 3)
Aug 03 17:00:10 btrbk_run.sh[2387275]: Finished un-mounting btrbk
btrtop volumes.
Aug 03 17:00:10 systemd[1]: mnt-btrtop-root.mount: Deactivated successfully.
Aug 03 18:00:06 btrbk_run.sh[2398488]: mounting btrbk btrtop volumes...

As mentioned, I have (temporarily) stopped unmounting these volumes
and I just leave them mounted all the time. The logs now look like
this:

Aug 06 03:00:14 btrbk_run.sh[3022708]: mounting btrbk btrtop volumes...
Aug 06 03:00:14 btrbk_run.sh[3022708]: INFO: [/mnt/btrtop/root] (1 of
3) was already mounted. Nothing to do.
Aug 06 03:00:14 btrbk_run.sh[3022708]: INFO: [/mnt/btrtop/home] (2 of
3) was already mounted. Nothing to do.
Aug 06 03:00:14 btrbk_run.sh[3022708]: INFO: [/mnt/btrtop/user] (3 of
3) was already mounted. Nothing to do.
Aug 06 03:00:14 btrbk_run.sh[3022708]: Finished mounting btrbk btrtop volumes.
Aug 06 04:00:14 btrbk_run.sh[3033520]: mounting btrbk btrtop volumes...
Aug 06 04:00:14 btrbk_run.sh[3033520]: INFO: [/mnt/btrtop/root] (1 of
3) was already mounted. Nothing to do.
Aug 06 04:00:14 btrbk_run.sh[3033520]: INFO: [/mnt/btrtop/home] (2 of
3) was already mounted. Nothing to do.
Aug 06 04:00:14 btrbk_run.sh[3033520]: INFO: [/mnt/btrtop/user] (3 of
3) was already mounted. Nothing to do.
Aug 06 04:00:14 btrbk_run.sh[3033520]: Finished mounting btrbk btrtop volumes.

>
> >
> > The path /var/cache/pacman is not a subvolume, but it resides on btrfs
> > subvolume @btrtop/snapshot. @btrtop/snapshot is normally mounted at
> > "/" but for btrfs tasks, it is also mounted at /mnt/btrtop/root. This
> > additional mount operation seems to be causing these nested mounts of
> > my bind mount for  /srv/nfs/var/cache/pacman .
> >
> > P.S. I cannot test without using systemd. (I'm not even sure I
> > remember how to use a non-systemd distro anymore!)
> >
>
>
> --
> gpg @keyserver.linux.it: Goffredo Baroncelli <kreijackATinwind.it>
> Key fingerprint BBF5 1610 0B64 DAC6 5F7D  17B2 0EDA 9B37 8B82 E0B5