Unmounting by filesystem ID

[Ian Dowse]

In message <1057413470.716.3.camel at localhost>, "Vladimir B. Grebenschikov" writ
es:
>May be you fix issue of umounting by known vnode ?
>problem is in mounting different devices under chroot.
>in list of mounts they differ only by device id, and it is rather=20
>difficult to umount filesystem if I known absolute path but do not enter
>to chroot.

The MNT_BYFSID approach makes it always possible to indicate to the
kernel which filesystem is to be unmounted, but some more work is
needed in the umount(8) utility to handle unusual cases.

The simplest way for umount to reliably pick the right file system
would be to have it call statfs(2) on the supplied path, and then
use the resulting filesystem ID to do the unmount(2) call. That
would allow you to unmount a filesystem that was mounted from within
a chroot by just specifying the full path to the filesystem even
though it is not the path that df or mount show.

Unfortunately, statfs(2) may get stuck if the path leads to (or
traverses) a network filesystem that is not responding. In this
case it is better for the umount utility to use getfsstat(2) with
the MNT_NOWAIT flag, and pick the filesystem that has the path that
was specified. This will do the wrong thing if for example you two
/var filesystems listed because one was mounted from inside a chroot.
There is also the problem of checking that the path points to the
root of the filesystem as statfs(2) works on any file or directory.

I think umount can probably be made do something reasonable in most
cases by having it use extra mechanisms such as statfs(2) only when
the named filesystem is not found on the mount list, or when it is
found multiple times, but it may be necessary to add a new command-line
argument to force it to use one of the two behaviours.

Note that you can also unmount a filesystem by specifying the device
name instead of the path. This works a bit better now than it used
to, as umount doesn't have to translate back into a path before
calling unmount(2). However it can still get a bit confused by
chroots:

	# chroot /root0 mdmfs -s 5m md0 /mnt
	# chroot /root1 mdmfs -s 5m md1 /mnt
	# df | grep md
	/dev/md0             4780        4    4396     0%    /mnt
	/dev/md1             4780        4    4396     0%    /mnt
	# umount /dev/md0
	umount: cannot umount /mnt, /dev/md1
		is mounted there, umount it first
	# df | grep md
	/dev/md0             4780        4    4396     0%    /mnt
	/dev/md1             4780        4    4396     0%    /mnt
	# umount -f /dev/md0
	# df | grep md
	/dev/md1             4780        4    4396     0%    /mnt
	# umount /dev/md1
	# df | grep md
	#

Ian