The 5.2 kernel saw the addition of an extensive new API for the mounting
(and remounting) of filesystems; this
article covered an early version of that API. Since then, work in this
area has mostly focused on enabling filesystems to support this API fully.
James Bottomley has taken a look at this API as part of the job of
redesigning his shiftfs filesystem and
found it to be incomplete. What has followed is a significant set of
changes that promise to simplify the mount API — though it turns out that
“simple” is often in the eye of the beholder.

Source: LWN.net – [$] configfd() and shifting bind mounts