Recursively unmount Bind mount in both User and Mount namespace

Question

I am trying to use recursive bind mount in a new user namespace and mount namespace.

This is what I do. Assume /foo/bar/ and /foo are both mount points. Firstly I do mount --rbind /foo/bar/ /baz, and then when I want to unmount it, I get

umount: /baz: device is busy.

I am sure that there is no other process is using /baz. Then I tried to unmount /baz/bar. This time I got

unmount: /baz/bar: not mounted.

However, I can see /baz/bar in /proc/mounts.

What is going wrong or how should I bind mount and then unmount in a new user & mount namespace?

Answer 1

You need umount -l which will unmount an entire tree in one system call.