[$] The end of the DAX experiment

Since its inception, the DAX mechanism (which provides for direct access to
files stored on persistent memory) has been seen as somewhat experimental
and incomplete. At the 2019 Linux Storage, Filesystem, and
Memory-Management Summit, Dan Williams ran a session where he said that
perhaps the time has come to end that experiment. Some of the
unimplemented DAX features may never actually need to be implemented, and
it might just be possible to declare DAX finished. But first there are a
few more details to take care of.