mm/z3fold.c: introduce helper functions

Patch series "z3fold: support page migration", v2.

This patchset implements page migration support and slightly better buddy
search.  To implement page migration support, z3fold has to move away from
the current scheme of handle encoding.  i.  e.  stop encoding page address
in handles.  Instead, a small per-page structure is created which will
contain actual addresses for z3fold objects, while pointers to fields of
that structure will be used as handles.

Thus, it will be possible to change the underlying addresses to reflect
page migration.

To support migration itself, 3 callbacks will be implemented:

1: isolation callback: z3fold_page_isolate(): try to isolate the page
   by removing it from all lists.  Pages scheduled for some activity and
   mapped pages will not be isolated.  Return true if isolation was
   successful or false otherwise

2: migration callback: z3fold_page_migrate(): re-check critical
   conditions and migrate page contents to the new page provided by the
   system.  Returns 0 on success or negative error code otherwise

3: putback callback: z3fold_page_putback(): put back the page if
   z3fold_page_migrate() for it failed permanently (i.  e.  not with
   -EAGAIN code).

To make sure an isolated page doesn't get freed, its kref is incremented
in z3fold_page_isolate() and decremented during post-migration compaction,
if migration was successful, or by z3fold_page_putback() in the other
case.

Since the new handle encoding scheme implies slight memory consumption
increase, better buddy search (which decreases memory consumption) is
included in this patchset.

This patch (of 4):

Introduce a separate helper function for object allocation, as well as 2
smaller helpers to add a buddy to the list and to get a pointer to the
pool from the z3fold header.  No functional changes here.

Link: http://lkml.kernel.org/r/20190417103633.a4bb770b5bf0fb7e43ce1666@gmail.com


Signed-off-by: Vitaly Wool <vitaly.vul@sony.com>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Cc: Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>
Cc: Uladzislau Rezki <urezki@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>