[PATCH] mm: micro-optimise rmap

Microoptimise page_add_anon_rmap.  Although these expressions are used only in
the taken branch of the if() statement, the compiler can't reorder them inside
because atomic_inc_and_test is a barrier.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 11 insertions, 10 deletions

diff --git a/mm/rmap.c b/mm/rmap.c
index f5a6966b7eb0..7e975ca24c78 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -439,22 +439,23 @@ int page_referenced(struct page *page, int is_locked, int ignore_token)
 void page_add_anon_rmap(struct page *page,
         struct vm_area_struct *vma, unsigned long address)
 {
-        struct anon_vma *anon_vma = vma->anon_vma;
-        pgoff_t index;
-
         BUG_ON(PageReserved(page));
-        BUG_ON(!anon_vma);
 
         inc_mm_counter(vma->vm_mm, anon_rss);
 
-        anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
-        index = (address - vma->vm_start) >> PAGE_SHIFT;
-        index += vma->vm_pgoff;
-        index >>= PAGE_CACHE_SHIFT - PAGE_SHIFT;
-
         if (atomic_inc_and_test(&page->_mapcount)) {
-                page->index = index;
+                struct anon_vma *anon_vma = vma->anon_vma;
+                pgoff_t index;
+
+                BUG_ON(!anon_vma);
+                anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
                 page->mapping = (struct address_space *) anon_vma;
+
+                index = (address - vma->vm_start) >> PAGE_SHIFT;
+                index += vma->vm_pgoff;
+                index >>= PAGE_CACHE_SHIFT - PAGE_SHIFT;
+                page->index = index;
+
                 inc_page_state(nr_mapped);
         }
         /* else checking page index and mapping is racy */