mm: rearrange page flags

Move PG_writeback into bottom byte so that it can use PG_waiters in a
later patch.  Move PG_head into bottom byte as well to match with where
'order' is moving next.  PG_active and PG_workingset move into the second
byte to make room for them.

By putting PG_head in bit 6, we ensure that it is cleared by assigning the
folio order to the bottom byte of the first tail page (since the order
cannot be larger than 63).

Link: https://lkml.kernel.org/r/20230816151201.3655946-10-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Cc: Yanteng Si <siyanteng@loongson.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/page-flags.h

@@ -99,13 +99,15 @@
  */
 enum pageflags {
 	PG_locked,		/* Page is locked. Don't touch. */
+	PG_writeback,		/* Page is under writeback */
 	PG_referenced,
 	PG_uptodate,
 	PG_dirty,
 	PG_lru,
+	PG_head,		/* Must be in bit 6 */
+	PG_waiters,		/* Page has waiters, check its waitqueue. Must be bit #7 and in the same byte as "PG_locked" */
 	PG_active,
 	PG_workingset,
-	PG_waiters,		/* Page has waiters, check its waitqueue. Must be bit #7 and in the same byte as "PG_locked" */
 	PG_error,
 	PG_slab,
 	PG_owner_priv_1,	/* Owner use. If pagecache, fs may use*/
@@ -113,8 +115,6 @@ enum pageflags {
 	PG_reserved,
 	PG_private,		/* If pagecache, has fs-private data */
 	PG_private_2,		/* If pagecache, has fs aux data */
-	PG_writeback,		/* Page is under writeback */
-	PG_head,		/* A head page */
 	PG_mappedtodisk,	/* Has blocks allocated on-disk */
 	PG_reclaim,		/* To be reclaimed asap */
 	PG_swapbacked,		/* Page is backed by RAM/swap */