As described above, pages are charged to their first touchers.
The first toucher is determined using pages' mapcount manipulations
in rmap calls.

Page is charged in two stages:
1. preparation, in which the resource availability is checked.
   This stage may lead to page reclamation, thus it is performed
   in a might-sleep places;
2. the container assignment to page. This is done in an atomic
   code that handles multiple touches.