On Tue, Jan 07, 2020 at 02:45:40PM -0800, John Hubbard wrote:

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.

I'm really not sold on this scheme.  Note that I think it is
particularly bad, but it also doesn't seem any better than what
we had before, and it introduced quite a bit more code.