[$] Some slow progress on get_user_pages()

One of the surest signs that the Linux Storage, Filesystem, and
Memory-Management (LSFMM) Summit is approaching is the seasonal migration of
memory-management developers toward the get_user_pages() problem.
This core kernel primitive is necessary for high-performance I/O to
user-space memory, but its interactions with filesystems have never been
reliable — or even fully specified. There are currently a couple of patch
sets in circulation that are attempting to improve the situation, though a
full solution still seems distant.