mm: CONFIG_NO_PAGE_MAPCOUNT to prepare for not maintain per-page mapcounts in large...

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.

Link: https://lkml.kernel.org/r/20250303163014.1128035-16-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Andy Lutomirks^H^Hski <luto@kernel.org>
Cc: Borislav Betkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Lance Yang <ioworker0@gmail.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Matthew Wilcow (Oracle) <willy@infradead.org>
Cc: Michal Koutn <mkoutny@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: tejun heo <tj@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Zefan Li <lizefan.x@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/Kconfig b/mm/Kconfig
index a25c5476b3ad44dcc44d43566324982e9c02ab87..4a4e7b63d30a88bdaaf0e897ed62d8c348c9d8e7 100644 (file)
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -887,8 +887,25 @@ 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 impact of some changes is still unclear.
+
 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
 #