mm: split page_type out from _mapcount

We're already using a union of many fields here, so stop abusing the
_mapcount and make page_type its own field.  That implies renaming some of
the machinery that creates PageBuddy, PageBalloon and PageKmemcg; bring
back the PG_buddy, PG_balloon and PG_kmemcg names.

As suggested by Kirill, make page_type a bitmask.  Because it starts out
life as -1 (thanks to sharing the storage with _mapcount), setting a page
flag means clearing the appropriate bit.  This gives us space for probably
twenty or so extra bits (depending how paranoid we want to be about
_mapcount underflow).

Link: http://lkml.kernel.org/r/20180518194519.3820-3-willy@infradead.org
Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Christoph Lameter <cl@linux.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Jérôme Glisse <jglisse@redhat.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 8 insertions, 5 deletions

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index eb9da245286d..3a554fdf45c2 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -96,6 +96,14 @@ struct page {
 	};
 
 	union {
+		/*
+		 * 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.
+		 */
+		unsigned int page_type;
+
 		_slub_counter_t counters;
 		unsigned int active;		/* SLAB */
 		struct {			/* SLUB */
@@ -109,11 +117,6 @@ struct page {
 			/*
 			 * Count of ptes mapped in mms, to show when
 			 * page is mapped & limit reverse map searches.
-			 *
-			 * Extra information about page type may be
-			 * stored here for pages that are never mapped,
-			 * in which case the value MUST BE <= -2.
-			 * See page-flags.h for more details.
 			 */
 			atomic_t _mapcount;