[systemd-devel] mount units and automatic (un)mounts

[Alad Wenter]

Hi,

When a device is mounted, a matching mount unit is created per
systemd.mount(5):

Mount points created at runtime (independently of unit files or
       /etc/fstab) will be monitored by systemd and appear like any other
       mount unit in systemd. See /proc/self/mountinfo description in
       proc(5).

If the device is removed while the mount is still active, systemd
unmounts the stale mount. However, if umount exits with a non-zero
status—for example, when the mount is still accessed and the drive
connection is flakey—the unit enters a failed state:

Jul 12 17:00:34 thinkpad systemd[1]: mnt-toshiba\x2dusb.mount: Mount
process exited, code=exited status=32
Jul 12 17:00:34 thinkpad systemd[1]: Failed unmounting /mnt/toshiba-usb.
Jul 12 17:00:34 thinkpad systemd[1]: mnt-toshiba\x2dusb.mount: Unit is
bound to inactive unit dev-sdc1.device. Stopping, too.
Jul 12 17:00:34 thinkpad systemd[1]: Unmounting /mnt/toshiba-usb...
Jul 12 17:00:34 thinkpad umount[6341]: umount: /mnt/toshiba-usb: target
is busy
Jul 12 17:00:34 thinkpad umount[6341]:         (In some cases useful
info about processes that
Jul 12 17:00:34 thinkpad umount[6341]:          use the device is found
by lsof(8) or fuser(1).)
Jul 12 17:00:34 thinkpad systemd[1]: mnt-toshiba\x2dusb.mount: Mount
process exited, code=exited status=32

On subsequent mounts, the device may be unmounted immediately unless
"systemctl reset-failed" is issued (note that appending the .mount unit
doesn't work in this case, giving a "Unit mnt-transcend\x2dsdhc.mount is
not loaded" error).

A few possibly related bug reports:

https://github.com/systemd/systemd/issues/2475
https://github.com/systemd/systemd/issues/1741
https://github.com/systemd/systemd/issues/1656

My attempt at a workaround is disabling systemd-fstab-generator per
systemd.generator(7), and creating a `mount -a` service which mounts
file systems at boot.

[Unit]
Description=Mount local filesystems
Documentation=man:mount(8) man:systemd.mount(5) man:bootup(7)
DefaultDependencies=No
After=local-fs-pre.target

[Service]
Type=oneshot
RemainAfterExit=yes
ExecStart=/bin/mount -a -t nocifs,nosmbfs,nonfs

[Install]
WantedBy=local-fs.target

However, according to the systemd.mount(5), .mount units will be
generated either way. Oddly enough, when using the above service they
don't seem to cause automatic mounts/unmounts.

How can this be the case? Is a better approach possible?

Regards,

Alad