[PATCH RFC 1/6] mm: update _mapcount and page_type documentation

[David Hildenbrand]

Let's make it clearer that _mapcount must no longer be used for own
purposes, and how _mapcount and page_type behaves nowadays (also in the
context of hugetlb folios, which are typed folios that will be mapped
to user space).

Move the documentation regarding "-1" over from page_mapcount_reset(),
which we will remove next.

Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
 include/linux/mm.h       |  5 -----
 include/linux/mm_types.h | 24 +++++++++++++++---------
 2 files changed, 15 insertions(+), 14 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 9849dfda44d4..018e7c0265ca 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1206,11 +1206,6 @@ static inline int folio_entire_mapcount(const struct folio *folio)
 	return atomic_read(&folio->_entire_mapcount) + 1;
 }
 
-/*
- * The atomic page->_mapcount, starts from -1: so that transitions
- * both from it and to it can be tracked, using atomic_inc_and_test
- * and atomic_add_negative(-1).
- */
 static inline void page_mapcount_reset(struct page *page)
 {
 	atomic_set(&(page)->_mapcount, -1);
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 24323c7d0bd4..532a3030405d 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -46,9 +46,7 @@ struct mem_cgroup;
  * which is guaranteed to be aligned.  If you use the same storage as
  * page->mapping, you must restore it to NULL before freeing the page.
  *
- * If your page will not be mapped to userspace, you can also use the four
- * bytes in the mapcount union, but you must call page_mapcount_reset()
- * before freeing it.
+ * The mapcount field must not be used for own purposes.
  *
  * If you want to use the refcount field, it must be used in such a way
  * that other CPUs temporarily incrementing and then decrementing the
@@ -152,16 +150,24 @@ struct page {
 
 	union {		/* This union is 4 bytes in size. */
 		/*
-		 * If the page can be mapped to userspace, encodes the number
-		 * of times this page is referenced by a page table.
+		 * For pages part of non-typed folios for which mappings are
+		 * tracked via the RMAP, encodes the number of times this page
+		 * is directly referenced by a page table.
+		 *
+		 * Note that the mapcount is always initialized to -1, so that
+		 * transitions both from it and to it can be tracked, using
+		 * atomic_inc_and_test() and atomic_add_negative(-1).
 		 */
 		atomic_t _mapcount;
 
 		/*
-		 * If the page is neither PageSlab nor mappable to userspace,
-		 * the value stored here may help determine what this page
-		 * is used for.  See page-flags.h for a list of page types
-		 * which are currently stored here.
+		 * For head pages of typed folios, the value stored here
+		 * allows for determining what this page is used for. The
+		 * tail pages of typed folios will not store a type
+		 * (mapcount == -1).
+		 *
+		 * See page-flags.h for a list of page types which are currently
+		 * stored here.
 		 */
 		unsigned int page_type;
 	};
-- 
2.45.0