mm: devmap: refactor 1-based refcounting for ZONE_DEVICE pages

mm: devmap: refactor 1-based refcounting for ZONE_DEVICE pages

An upcoming patch changes and complicates the refcounting and especially
the "put page" aspects of it.  In order to keep everything clean,
refactor the devmap page release routines:

* Rename put_devmap_managed_page() to page_is_devmap_managed(), and
  limit the functionality to "read only": return a bool, with no side
  effects.

* Add a new routine, put_devmap_managed_page(), to handle decrementing
  the refcount for ZONE_DEVICE pages.

* Change callers (just release_pages() and put_page()) to check
  page_is_devmap_managed() before calling the new
  put_devmap_managed_page() routine.  This is a performance point:
  put_page() is a hot path, so we need to avoid non- inline function calls
  where possible.

* Rename __put_devmap_managed_page() to free_devmap_managed_page(), and
  limit the functionality to unconditionally freeing a devmap page.

This is originally based on a separate patch by Ira Weiny, which applied
to an early version of the put_user_page() experiments.  Since then,
Jérôme Glisse suggested the refactoring described above.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ira Weiny <[email protected]>
Signed-off-by: John Hubbard <[email protected]>
Suggested-by: Jérôme Glisse <[email protected]>
Reviewed-by: Dan Williams <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Alex Williamson <[email protected]>
Cc: Aneesh Kumar K.V <[email protected]>
Cc: Björn Töpel <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Hans Verkuil <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Leon Romanovsky <[email protected]>
Cc: Mauro Carvalho Chehab <[email protected]>
Cc: Mike Rapoport <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>