[PATCH] fix extra page ref count in follow_hugetlb_page

git-commit: d5d4b0aa
"[PATCH] optimize follow_hugetlb_page" breaks mlock on hugepage areas.

I mis-interpret pages argument and made get_page() unconditional.  It
should only get a ref count when "pages" argument is non-null.

Credit goes to Adam Litke who spotted the bug.
Signed-off-by: Ken Chen <kenneth.w.chen@intel.com>
Acked-by: Adam Litke <agl@us.ibm.com>
Cc: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/hugetlb.c

@@ -697,9 +697,10 @@ int follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 		pfn_offset = (vaddr & ~HPAGE_MASK) >> PAGE_SHIFT;
 		page = pte_page(*pte);
 same_page:
-		get_page(page);
-		if (pages)
+		if (pages) {
+			get_page(page);
 			pages[i] = page + pfn_offset;
+		}
 
 		if (vmas)
 			vmas[i] = vma;