userfaultfd: add new syscall to provide memory externalization

userfaultfd: add new syscall to provide memory externalization

Once an userfaultfd has been created and certain region of the process
virtual address space have been registered into it, the thread responsible
for doing the memory externalization can manage the page faults in
userland by talking to the kernel using the userfaultfd protocol.

poll() can be used to know when there are new pending userfaults to be
read (POLLIN).

Signed-off-by: Andrea Arcangeli <[email protected]>
Acked-by: Pavel Emelyanov <[email protected]>
Cc: Sanidhya Kashyap <[email protected]>
Cc: [email protected]
Cc: "Kirill A. Shutemov" <[email protected]>
Cc: Andres Lagar-Cavilla <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Peter Feiner <[email protected]>
Cc: "Dr. David Alan Gilbert" <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: "Huangpeng (Peter)" <[email protected]>
Cc: Dan Carpenter <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>