[$] Tracking pages from get_user_pages()

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