hugetlb, rmap: always use anon_vma root pointer

This patch applies Andrea's fix given by the following patch into hugepage
rmapping code:

  commit 288468c3
  Author: Andrea Arcangeli <aarcange@redhat.com>
  Date:   Mon Aug 9 17:19:09 2010 -0700

This patch uses anon_vma->root and avoids unnecessary overwriting when
anon_vma is already set up.
Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Acked-by: Andrea Arcangeli <aarcange@redhat.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/rmap.c

@@ -1564,13 +1564,14 @@ static void __hugepage_set_anon_rmap(struct page *page,
 	struct vm_area_struct *vma, unsigned long address, int exclusive)
 {
 	struct anon_vma *anon_vma = vma->anon_vma;
+
 	BUG_ON(!anon_vma);
-	if (!exclusive) {
-		struct anon_vma_chain *avc;
-		avc = list_entry(vma->anon_vma_chain.prev,
-				 struct anon_vma_chain, same_vma);
-		anon_vma = avc->anon_vma;
-	}
+
+	if (PageAnon(page))
+		return;
+	if (!exclusive)
+		anon_vma = anon_vma->root;
+
 	anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
 	page->mapping = (struct address_space *) anon_vma;
 	page->index = linear_page_index(vma, address);