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

[Christoph Hellwig <hch@infradead.org>]

On Fri, Nov 03, 2023 at 04:47:02PM +0100, Christian Brauner wrote:
> I think the idea of using vfsmounts for this makes some sense if the
> goal is to retroactively justify and accommodate the idea that a
> subvolume is to be treated as equivalent to a separate device.

st_dev has only been very historically about treating something as
a device.  For userspae the most important part is that it designates
a separate domain for inode numbers.  And that's something that's simply
broken in btrfs.

> I question that premise though. I think marking them with separate
> device numbers is bringing us nothing but pain at this point and this
> solution is basically bending the vfs to make that work somehow.

Well, the only other theoretical option would be to use a simple
inode number space across subvolumes in btrfs, but I don't really
see how that could be retrofitted in any sensible way.

> I would feel much more comfortable if the two filesystems that expose
> these objects give us something like STATX_SUBVOLUME that userspace can
> raise in the request mask of statx().

Except that this doesn't fix any existing code.

> If userspace requests STATX_SUBVOLUME in the request mask, the two
> filesystems raise STATX_SUBVOLUME in the statx result mask and then also
> return the _real_ device number of the superblock and stop exposing that
> made up device number.

What is a "real" device number?