umount: target is busy

Question

I have mounted /dev and immediately tried to unmount:

$ sudo mount -o rbind /dev m
$ sudo umount m
umount: /tmp/m: target is busy.
$ sudo lsof m
lsof: WARNING: can't stat() fuse.gvfsd-fuse file system /run/user/1000/gvfs
      Output information may be incomplete.
COMMAND   PID USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
kdevtmpfs  55 root  cwd    DIR    0,6     4420    2 m
kdevtmpfs  55 root  rtd    DIR    0,6     4420    2 m

I have read that fuser can kill processes accessing the mount point, but I would like to understand what is happening in this simple case. Acording to the lsof output does something use the mountpoint as current working directory (cwd)?

I do not want to use lazy unmount.

Answer 1

You used rbind to mount a filesystem and submounts. In order to unmount a filesystem, you must unmount its submounts first (and the same for their submounts, recursively). But take care!

mount --make-rslave m
umount -R m

Without the first command, you risk unmounting all the sub-mounts on the source, due to mount propagation. In this case that means all the sub-mounts of /dev, which would have bad effects on your running system ;-).

Basically mount propagation is a massive pit-trap waiting for you to fall into it :-). It seems like it would have been better if bind mounts disabled it by default.

---

kdevtmpfs is the kernel thread that maintains devtmpfs. It does not prevent unmounting devtmpfs. This is because the kernel thread runs on a separate mount (like a bind mount). You can't see that original mount; it is in a separate mount namespace. If you want to try and work out why kdevtmpfs shows up in lsof, I don't know, maybe consider that a separate question.