Re: [PATCH 0/3] fanotify support for btrfs sub-volumes

[Christoph Hellwig <hch@infradead.org>]

On Wed, Nov 01, 2023 at 09:16:50AM +0100, Christian Brauner wrote:
> mkfs.btrfs -f /dev/sda
> mount -t btrfs /dev/sda /mnt
> btrfs subvolume create /mnt/subvol1
> btrfs subvolume create /mnt/subvol2
> 
> Then all subvolumes are always visible under /mnt.
> IOW, you can't hide them other than by overmounting or destroying them.

Yes.

> If we make subvolumes vfsmounts then we very likely alter this behavior
> and I see two obvious options:
> 
> (1) They are fake vfsmounts that can't be unmounted:
> 
>     umount /mnt/subvol1 # returns -EINVAL
> 
>     This retains the invariant that every subvolume is always visible
>     from the filesystems root, i.e., /mnt will include /mnt/subvol{1,}

Why would we have to prevent them to be automounted?  I'd expect
automount-like behavior where they are automatially mounted and then
expired or manuall unmounted.

> But if we do e.g., (2) then this surely needs to be a Kconfig and/or a
> mount option to avoid breaking userspace (And I'm pretty sure that btrfs
> will end up supporting both modes almost indefinitely.).

It would definitively need to be an opt-in for existing systems.