On Thu, Jul 15, 2021 at 01:11:29PM -0400, Josef Bacik wrote:

Because there's no alternative.  We need a way to tell userspace they've
wandered into a different inode namespace.  There's no argument that what
we're doing is ugly, but there's never been a clear "do X instead".  Just a
lot of whinging that btrfs is broken.  This makes userspace happy and is
simple and straightforward.  I'm open to alternatives, but there have been 0
workable alternatives proposed in the last decade of complaining about it.

Make sure we cross a vfsmount when crossing the "st_dev" domain so
that it is properly reported.   Suggested many times and ignored all
the time beause it requires a bit of work.