How to unmount a formerly chroot'd filesystem?

Question

I'm using a rescue-live-system (similar to a live-cd) to fix some issues with my Debian server, like that:

# mkdir -p /mnt/rescue
# mount /dev/md2 /mnt/rescue
# mount --bind /dev /mnt/rescue/dev/
# mount --bind /proc /mnt/rescue/proc/
# mount --bind /sys /mnt/rescue/sys/

Now I can chroot to /mnt/rescue - but after I'm done, how to unmount the filesystem again?

umount: /mnt/rescue: target is busy.
    (In some cases useful info about processes that use
     the device is found by lsof(8) or fuser(1))

I guess it's because dev, proc and sys are bound to the mounted file system. But it's not possible to unmount them either...

Answer 1

1. You have to first exit the chroot session, usually a simple exit will do:

   exit
   

2. Then umount ALL binded directories:

   umount /mnt/rescue/dev/
   umount /mnt/rescue/proc/
   umount /mnt/rescue/sys/
   

3. Then:

   umount /mnt/rescue
   

---

In case you were worried that sync isn't used here, note that it has no influence on whether unmounting is possible. Unmounting flushes pending writes anyway (it has to, because there'd be nowhere for them to go after the unmounting). The presence of a chrooted process is irrelevant (except in that it prevents unmounting). In normal system operation, sync has no observable effect. sync only makes a difference if a device is physically disconnected without having been unmounted or if the system crashes while the device is mounted.

Answer 2

Execute the below command to force and Detach the filesystem from the filesystem hierarchy, and cleanup all references to the filesystem as soon as it is not busy anymore.

umount -lf /mnt/rescue

Answer 3

The reason why you get the 'target is busy.' message is because the mount point (/mnt/rescue) is open in a file browser or in a terminal session, and also the order of unmounting process (here I mean dev/pts should be umounted before dev/ )

Well, in order to successfully umount all fs there :

• Make sure the mountpoint isn't open in a file browser!
• After exiting chroot change directory out of chroot dir (cd)!

• Umount fs respecting the order dev/pts => dev/ => proc/ => sys/ :

  sudo umount /mnt/rescue/dev/pts
  sudo umount /mnt/rescue/dev
  sudo umount /mnt/rescue/proc
  sudo umount /mnt/rescue/sys
  sudo umount /mnt/rescue

Answer 4

This is how I do with schroot command on Ubuntu version 10.04 upward:

# list all sessions:
schroot --list --all-sessions
# if the above command does not work, just type `mount`. The bind mount
# points with something like this in the mount path is the session name you want to get:
precise-a4aac8e0-663c-4fec-8fb2-16e4b06557e3 (<chroot_name>-<id>)

# now run this to properly end the session:
schroot -e -c precise-ca6c72e4-0e9f-4721-8a0e-cca359e2c2fd

Answer 5

Exit chroot. In the host system, the command 'mount' will show all the mounted path. (Includes those path which are mounted in chroot.) For example:

binfmt_misc on /home/user/projects/jsroot/proc/sys/fs/binfmt_misc

Then enter chroot environment. In chroot environment run unmount to unmount all path in order. (Sub path must be unmount before parent path.)

unmount /proc/sys/fs/binfmt_misc
unmount /proc/sys
unmount /proc