[PATCH RFC 0/6] mm: page_type, zsmalloc and page_mapcount_reset()

[David Hildenbrand]

Wanting to remove the remaining abuser of _mapcount/page_type along with
page_mapcount_reset(), I stumbled over zsmalloc, which is yet to be
converted away from "struct page" [1].

Unfortunately, we cannot stop using the page_type field in zsmalloc code
completely for its own purposes. All other fields in "struct page" are
used one way or the other.

.. but we can limit the abuse to 16 bit, glue it to a apge type that
must be set, and document it. page_has_type() will always successfully
indicate such zsmalloc pages, and such zsmalloc pages only.

We lose zsmalloc support for PAGE_SIZE > 64KB, which should be tolerable.
We could use more bits from the page type, but 16 bit sounds like a good
idea.

So clarify the _mapcount/page_type documentation, use a proper page_type
for zsmalloc, and remove page_mapcount_reset().

Only lightly tested with zram. Will have to do more testing and
cross-compile checking.

[1] https://lore.kernel.org/all/20231130101242.2590384-1-42.hyeyoo@xxxxxxxxx/

Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
Cc: Mike Rapoport <rppt@xxxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
Cc: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>

David Hildenbrand (6):
  mm: update _mapcount and page_type documentation
  mm: allow reuse of the lower 16bit of the page type with an actual
    type
  mm/zsmalloc: use a proper page type
  mm/page_alloc: clear PageBuddy using __ClearPageBuddy() for bad pages
  mm/filemap: reinitialize folio->_mapcount directly
  mm/mm_init: initialize page->_mapcount directly in__init_single_page()

 include/linux/mm.h         | 10 ----------
 include/linux/mm_types.h   | 29 ++++++++++++++++++++---------
 include/linux/page-flags.h | 25 +++++++++++++++++--------
 mm/Kconfig                 |  1 +
 mm/filemap.c               |  2 +-
 mm/mm_init.c               |  2 +-
 mm/page_alloc.c            |  6 ++++--
 mm/zsmalloc.c              | 23 +++++++++++++++++++----
 8 files changed, 63 insertions(+), 35 deletions(-)


base-commit: 29c73fc794c83505066ee6db893b2a83ac5fac63
-- 
2.45.0