[PATCH v2 15/20] mm: CONFIG_NO_PAGE_MAPCOUNT to prepare for not maintain per-page mapcounts in large folios

David Hildenbrand
2025-02-24T16:56:53Z

We're close to the finishing line: let's introduce a new
CONFIG_NO_PAGE_MAPCOUNT config option where we will incrementally remove
any dependencies on per-page mapcounts in large folios. Once that's
done, we'll stop maintaining the per-page mapcounts with this
config option enabled.

CONFIG_NO_PAGE_MAPCOUNT will be EXPERIMENTAL for now, as we'll have to
learn about some of the real world impact of some of the implications.

As writing "!CONFIG_NO_PAGE_MAPCOUNT" is really nasty, let's introduce
a helper config option "CONFIG_PAGE_MAPCOUNT" that expresses the
negation.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
mm/Kconfig | 18 ++++++++++++++++++
1 file changed, 18 insertions(+)

diff --git a/mm/Kconfig b/mm/Kconfig
index af72fe92f96b0..64bb80a281dd9 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -889,8 +889,26 @@ config READ_ONLY_THP_FOR_FS
support of file THPs will be developed in the next few release
cycles.

+config NO_PAGE_MAPCOUNT
+ bool "No per-page mapcount (EXPERIMENTAL)"
+ help
+ Do not maintain per-page mapcounts for pages part of larger
+ allocations, such as transparent huge pages.
+
+ When this config option is enabled, some interfaces that relied on
+ this information will rely on less-precise per-allocation information
+ instead: for example, using the average per-page mapcount in such
+ a large allocation instead of the per-page mapcount.
+
+ EXPERIMENTAL because the severity of some of the implications first
+ have to be understood properly.
+
endif # TRANSPARENT_HUGEPAGE

+# simple helper to make the code a bit easier to read
+config PAGE_MAPCOUNT
+ def_bool !NO_PAGE_MAPCOUNT
+
#
# The architecture supports pgtable leaves that is larger than PAGE_SIZE
#
--
2.48.1