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>
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 8c149fa..695b904 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -269,14 +269,7 @@
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 int compound_order(struct page *page)
{
- if (!PageCompound(page) || PageTail(page))
+ if (!PageHead(page))
return 0;
return (unsigned long)page[1].lru.prev;
}
