[$] DAX semantics

<p>
In the filesystems track at the 2019 Linux Storage, Filesystem, and
Memory-Management Summit, Ted Ts'o led a discussion about an inode flag to
indicate DAX files, which is meant to be applied to files that should be
directly accessed without going through the page cache. XFS has such a
flag, but ext4 and
other filesystems do not. The semantics of what the flag would mean are
not clear to Ts'o (and probably others), so the intent of the discussion
was to try to nail those down.