Trying to use a chrooted system via

mount -B stage3 gentoo
mount -t sysfs none gentoo/sys
mount -t proc none gentoo/proc
mount -R /dev gentoo/dev
mount --make-rslave gentoo/dev
unshare --fork chroot gentoo
umount -R gentoo

After the last umount, dev, dev/pts, dev/shm, dev/mqueue, dev/hugepages were still mounted under stage3.

I have two questions,

  1. Why they were mounted to stage3?
  2. How could I unmount those? Every mountpoint is busy.

OS: Ubuntu Xenial. Kernel: 4.13.4, but I have got same issue with kernel like 4.10 and others.

  1. Why they were mounted to stage3?

    Because you bind-mounted stage3 onto gentoo and stage3 propagation flag is set to shared (verify this with: findmnt -o PROPAGATION stage3)

  2. How could I unmount those? Every mountpoint is busy.

    You've just spawned a chroot on the mounted tree. If you want to unmount the tree while the chroot command is running, make the mount namespace private to the chroot:

     unshare --mount --fork chroot gentoo
| improve this answer | |
  • (and the default to shared propagation is probably due to systemd: unix.stackexchange.com/questions/269695/…) – sourcejedi Oct 7 '17 at 17:52
  • Thanks for the explanation. Shared sub-tree is always confusing me. For question 2, I ran umount after exited the chroot jail. umount -R unmounted gentoo, sys, proc properly. But dev and its children did not get unmounted due to "busy". – Magicloud Oct 8 '17 at 5:44
  • Sorry last comment was not editable. I mean dev and its children (on stage3) did not get unmounted due to "busy". Let me check the share part since dev on gentoo got unmounted. – Magicloud Oct 8 '17 at 6:06
  • @Magicloud It did not get unmounted with the --mount option? Use lsof to find out what is using it. – xhienne Oct 8 '17 at 9:40
  • Oh, --mount works here. And --make-private when binding stage3 to gentoo also works. – Magicloud Oct 9 '17 at 2:42

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.