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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...

q9f
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5 Answers5

23

  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.

tshepang
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John Siu
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    thanks, that's it... my mistake was trying to unmount /sys/ instead of /mnt/rescue/sys/ ... – q9f Jan 20 '13 at 17:21
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    The `sync` is totally useless. – Gilles 'SO- stop being evil' Jan 20 '13 at 22:54
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    @Gilles Can you elaborate abit more why `sync` is useless? Does it become useless in current kernel? Or only in this case(rescue mode)? I put it there just in case there are huge pending write to disk that will prevent umount after exiting chroot. – John Siu Jan 21 '13 at 02:44
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    @JohnSiu `sync` 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. – Gilles 'SO- stop being evil' Jan 21 '13 at 10:35
  • @Gilles I understand your point now. Thank you!! – John Siu Jan 21 '13 at 10:50
20

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
shgnInc
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  • there's no comment here regarding the nested mounts. does this clean them up safely? can you unmount a filesystem contianing a chroot after this, if all processes have terminated? – fuzzyTew Oct 31 '20 at 10:40
  • Be careful with this one when using recursive mounts, e.g: `--rbind`. Subsequent recursive mounts of `/sys` and `/dev` don't mount the entire tree! – Saad Malik Feb 14 '21 at 16:58
4

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

Yunus
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3

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
biocyberman
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  • perfect thx! on the `schroot --help` "-c [ --chroot ] arg Use specified chroot" it should actually read: "-c [ --chroot ] arg Use specified chroot SESSION"! ubuntu 20.04 here – Aquarius Power Dec 02 '20 at 20:46
  • @AquariusPower in 2020, wonder why docker doesn’t work for you :-? – biocyberman Dec 02 '20 at 20:50
  • do you think docker could work for a i386 install that can be integrated on the running X server? https://askubuntu.com/questions/1274951/how-to-successfully-install-wine-with-win32-i386-support-on-ubuntu-20-0 – Aquarius Power Dec 02 '20 at 22:03
  • @AquariusPower I don’t know how far you want to go with schroot and docker, but this might be helpful: [x11docker](https://github.com/mviereck/x11docker) – biocyberman Dec 02 '20 at 22:18
  • cool thx! I would have to read more but it seems to run a new X server (inside current one?) but the main point would be security. What I am doing I dont really know if it is as safe as x11docker approach, but what I am doing looks more integrated (like the app looks to be running not from a chroot at all). The whole point, in the end, will be if the applications I want to run will properly run on x11docker as well as I believe they will with schroot (as I havent been able to fully test it yet xD). – Aquarius Power Dec 02 '20 at 22:58
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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
PokerFace
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