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

[David Hildenbrand <david@redhat.com>]

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