[PATCH v2 1/6] mm: update _mapcount and page_type documentation

David Hildenbrand
2024-05-29T11:19:14Z

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. Move "page_type" before "mapcount", to make
it clearer what typed folios are.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
include/linux/mm.h | 5 -----
include/linux/mm_types.h | 28 +++++++++++++++++-----------
2 files changed, 17 insertions(+), 16 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 3aa1b6889bccf..eebfce8f58bca 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 24323c7d0bd48..dd2ce1b3ec80e 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
@@ -152,18 +150,26 @@ 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 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
+ * (page_type == _mapcount == -1).
+ *
+ * See page-flags.h for a list of page types which are currently
+ * stored here.
*/
- atomic_t _mapcount;
+ unsigned int page_type;

/*
- * 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 pages that are 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).
*/
- unsigned int page_type;
+ atomic_t _mapcount;
};

/* Usage count. *DO NOT USE DIRECTLY*. See page_ref.h */
--
2.45.1