[$] Tracking pages from get_user_pages()

As has been recently discussed here,
developers for the filesystem and memory-management subsystems have been
grappling for years with the problems posed by the get_user_pages()
mechanism. This function maps memory into the kernel's address space for
direct access by the kernel or peripheral devices, but that kind of access
can create confusion in the filesystem layers, which may not be expecting
that memory to be written to at any given time. A new patch
set from Jérôme Glisse tries to chip away at a piece of the problem,
but a complete solution is not yet in view.