mm: update _mapcount and page_type documentation

Patch series "mm: page_type, zsmalloc and page_mapcount_reset()", v2.

Wanting to remove the remaining abuser of _mapcount/page_type along with
page_mapcount_reset(), I stumbled over zsmalloc, which is yet to be
converted away from "struct page" [1].

Unfortunately, we cannot stop using the page_type field in zsmalloc code
completely for its own purposes.  All other fields in "struct page" are
used one way or the other.  Could we simply store a 2-byte offset value at
the beginning of each page?  Likely, but that will require a bit more
work; and once we have memdesc we might want to move the offset in there
(struct zsalloc?) again.

...  but we can limit the abuse to 16 bit, glue it to a page type that
must be set, and document it.  page_has_type() will always successfully
indicate such zsmalloc pages, and such zsmalloc pages only.

We lose zsmalloc support for PAGE_SIZE > 64KB, which should be tolerable. 
We could use more bits from the page type, but 16 bit sounds like a good
idea for now.

So clarify the _mapcount/page_type documentation, use a proper page_type
for zsmalloc, and remove page_mapcount_reset().

[1] https://lore.kernel.org/all/20231130101242.2590384-1-42.hyeyoo@gmail.com/


This patch (of 6):

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.

Link: https://lkml.kernel.org/r/20240529111904.2069608-1-david@redhat.com
Link: https://lkml.kernel.org/r/20240529111904.2069608-2-david@redhat.comSigned-off-by: David Hildenbrand <david@redhat.com>
Tested-by: Sergey Senozhatsky <senozhatsky@chromium.org>	[zram/zsmalloc workloads]
Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/mm.h

@@ -1210,11 +1210,6 @@ 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);

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 */