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, 4 insertions, 6 deletions

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index fc241fe295ab..36d713e216e8 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -235,12 +235,11 @@ static void prep_compound_page(struct page *page, unsigned long order)
 
         set_compound_page_dtor(page, free_compound_page);
         set_compound_order(page, order);
-        __SetPageCompound(page);
+        __SetPageHead(page);
         for (i = 1; i < nr_pages; i++) {
                 struct page *p = page + i;
 
                 __SetPageTail(p);
-                __SetPageCompound(p);
                 p->first_page = page;
         }
 }
@@ -253,17 +252,16 @@ static void destroy_compound_page(struct page *page, unsigned long order)
         if (unlikely(compound_order(page) != order))
                 bad_page(page);
 
-        if (unlikely(!PageCompound(page)))
+        if (unlikely(!PageHead(page)))
                         bad_page(page);
-        __ClearPageCompound(page);
+        __ClearPageHead(page);
         for (i = 1; i < nr_pages; i++) {
                 struct page *p = page + i;
 
-                if (unlikely(!PageCompound(p) | !PageTail(p) |
+                if (unlikely(!PageTail(p) |
                                 (p->first_page != page)))
                         bad_page(page);
                 __ClearPageTail(p);
-                __ClearPageCompound(p);
         }
 }