[PATCH v2 2/5] mm: Move PG_writeback into the bottom byte

[Matthew Wilcox <willy@infradead.org>]

From: "Matthew Wilcox (Oracle)" <willy@infradead.org>

By placing PG_writeback in the bottom byte along with PG_locked and
PG_waiters, we will be able to use the same optimisation as PG_locked
in a subsequent patch.  Add BUILD_BUGs to ensure that all three bits
stay in the bottom byte.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
---
 include/linux/page-flags.h | 4 ++--
 mm/filemap.c               | 4 +++-
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 222f6f7b2bb3..af7c0ff5f517 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -103,13 +103,14 @@
  */
 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_active,
+	PG_waiters,		/* Page has waiters, check its waitqueue */
 	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*/
@@ -117,7 +118,6 @@ 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 */
diff --git a/mm/filemap.c b/mm/filemap.c
index e475117e89eb..b7c5d2402370 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1266,6 +1266,7 @@ EXPORT_SYMBOL_GPL(add_page_wait_queue);
 void unlock_page(struct page *page)
 {
 	BUILD_BUG_ON(PG_waiters != 7);
+	BUILD_BUG_ON(PG_locked > 7);
 	page = compound_head(page);
 	VM_BUG_ON_PAGE(!PageLocked(page), page);
 	if (clear_bit_unlock_is_negative_byte(PG_locked, &page->flags))
@@ -1279,12 +1280,13 @@ EXPORT_SYMBOL(unlock_page);
  */
 void end_page_writeback(struct page *page)
 {
+	BUILD_BUG_ON(PG_writeback > 7);
 	/*
 	 * TestClearPageReclaim could be used here but it is an atomic
 	 * operation and overkill in this particular case. Failing to
 	 * shuffle a page marked for immediate reclaim is too mild to
 	 * justify taking an atomic operation penalty at the end of
-	 * ever page writeback.
+	 * every page writeback.
 	 */
 	if (PageReclaim(page)) {
 		ClearPageReclaim(page);
-- 
2.25.1