[$] get_user_pages(), pinned pages, and DAX

The problems associated with the kernel's internal get_user_pages()function have been a topic of discussion at the Linux Storage, Filesystem,and Memory-Management Summit for a few years. At the 2019 event, Jan Kara began a plenary session by saying that it would be "like lastyear's session". It turned out rather differently, though, perhaps due tothe plenary setting; this discussion (along with the related session thatfollowed) turned out to be one of the most heated at the entire conference.