Bind-mount remains busy after exiting from chroot

Question

I had to chroot into an offline system for debugging:

mount /dev/foo/root /media/chroot
mount -t proc /proc /media/chroot/proc
mount --rbind /sys /media/chroot/sys
mount --rbind /dev /media/chroot/dev
chroot /media/chroot /bin/bash

This worked fine. However, after exiting from the chroot I'm unable to remove the /dev and /sys bind-mounts. umount complains that the target is still busy:

root@localhost:~# umount /media/chroot/sys
umount: /media/chroot/sys: target is busy
        (In some cases useful info about processes that
         use the device is found by lsof(8) or fuser(1).)

fuser -m /media/chroot/sys shows several processes (including init) still accessing that mountpoint, and I don't think I should kill all those processes, and I don't want to force a umount unless it's absolutely necessary.

Is there a "normal" way to umount bind-mounted directories?

Answer 1

Umounting a bind-mounted filesystem is usually no different from umounting any other filesystem. However, in your case you used the option --rbind instead of just --bind with the /dev and /sys filesystems, which means you did a recursive mount. From man mount:

-R, --rbind
Remount a subtree and all possible submounts somewhere else (so that its contents are available in both places). See above.

Both /dev and /sys are not just single filesystems, but have several other things mounted in them (for example there are several cgroups mounted below /sys), which the recursive mount replicates under your chroot mountpoint.

root@localhost:~# mount | grep 'chroot/sys'
sysfs on /media/chroot/sys type sysfs (rw,nosuid,nodev,noexec,relatime)
none on /media/chroot/sys/fs/cgroup type tmpfs (rw,relatime,size=4k,mode=755)
cgroup on /media/chroot/sys/fs/cgroup/cpuset type cgroup (rw,relatime,cpuset)
cgroup on /media/chroot/sys/fs/cgroup/cpu type cgroup (rw,relatime,cpu)
cgroup on /media/chroot/sys/fs/cgroup/cpuacct type cgroup (rw,relatime,cpuacct)
cgroup on /media/chroot/sys/fs/cgroup/blkio type cgroup (rw,relatime,blkio)
cgroup on /media/chroot/sys/fs/cgroup/memory type cgroup (rw,relatime,memory)
...

These additional mounts are not automatically removed along with their ancestor and are what's preventing you from simply umounting /media/chroot/dev and /media/chroot/sys. You need to umount all of those first before you can remove the /dev and /sys bind-mounts.

...
umount /media/chroot/sys/fs/cgroup/cpu
umount /media/chroot/sys/fs/cgroup/cpuset
umount /media/chroot/sys/fs/cgroup
umount /media/chroot/sys
umount /media/chroot/dev/pts
umount /media/chroot/dev