[$] Write-protect for userfaultfd()

The userfaultfd()
system call allows one process to handle page faults for another — in user
space. Its original use case was to support transparent container
migration, but other uses have developed over the years. At the 2019 Linux
Storage, Filesystem, and Memory-Management Summit, Andrea Arcangeli
described a scheme to add write-protection
support to userfaultfd(). After a year of lost time fighting
speculative-execution problems, Arcangeli is about ready to move this feature
into the mainline.