[PATCH v1 0/6] mm: page_type, zsmalloc and page_mapcount_reset()

From: David Hildenbrand
Date: Mon May 27 2024 - 10:28:27 EST


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. Could we simply store a 2-byte offset value
at the beginning of each page? Likely, but that will require a bit more
work; and once we have memdesc we might want to move the offset in there
(struct zsalloc?) again.

.. but we can limit the abuse to 16 bit, glue it to a page 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 for now.

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

Survived a couple of days with the built bots and my cross-compile
attempts. Briefly tested with zram on x86-64.

[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>

v1 -> v2:
* Rebased to v6.10-rc1
* "mm: update _mapcount and page_type documentation"
-> Exchange members and fixup doc as suggested by Mike
* "mm: allow reuse of the lower 16bit of the page type with an actual
type"
-> Remove "highest bit" comment, fixup PG_buddy, extend description
* "mm/zsmalloc: use a proper page type"
-> Add and use HAVE_ZSMALLOC to fixup compilcation
-> Fixup BUILD_BUG_ON
-> Add some VM_WARN_ON_ONCE(!PageZsmalloc(page));
* "mm/mm_init: initialize page->_mapcount directly
in __init_single_page()"
-> Fixup patch subject

David Hildenbrand (6):
mm: update _mapcount and page_type documentation
mm: allow reuse of the lower 16 bit 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()

drivers/block/zram/Kconfig | 1 +
include/linux/mm.h | 10 ----------
include/linux/mm_types.h | 33 ++++++++++++++++++++++-----------
include/linux/page-flags.h | 23 +++++++++++++++--------
mm/Kconfig | 10 ++++++++--
mm/filemap.c | 2 +-
mm/mm_init.c | 2 +-
mm/page_alloc.c | 6 ++++--
mm/zsmalloc.c | 29 +++++++++++++++++++++++++----
9 files changed, 77 insertions(+), 39 deletions(-)


base-commit: 1613e604df0cd359cf2a7fbd9be7a0bcfacfabd0
--
2.45.1