mm: optimize compound_head() by avoiding a shared page flag

The patch adds PageTail(page) and PageHead(page) to check if a page is the
head or the tail of a compound page.  This is done by masking the two bits
describing the state of a compound page and then comparing them.  So one
comparision and a branch instead of two bit checks and two branches.

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 9 deletions

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 8c149fa4491d..695b90437bbc 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -269,14 +269,7 @@ static inline int get_page_unless_zero(struct page *page)
 
 static inline struct page *compound_head(struct page *page)
 {
-	/*
-	 * We could avoid the PageCompound(page) check if
-	 * we would not overload PageTail().
-	 *
-	 * This check has to be done in several performance critical
-	 * paths of the slab etc. IMHO PageTail deserves its own flag.
-	 */
-	if (unlikely(PageCompound(page) && PageTail(page)))
+	if (unlikely(PageTail(page)))
 		return page->first_page;
 	return page;
 }
@@ -327,7 +320,7 @@ static inline compound_page_dtor *get_compound_page_dtor(struct page *page)
 
 static inline int compound_order(struct page *page)
 {
-	if (!PageCompound(page) || PageTail(page))
+	if (!PageHead(page))
 		return 0;
 	return (unsigned long)page[1].lru.prev;
 }