hugetlb: correct page count for surplus huge pages

Free pages in the hugetlb pool are free and as such have a reference count of
zero.  Regular allocations into the pool from the buddy are "freed" into the
pool which results in their page_count dropping to zero.  However, surplus
pages can be directly utilized by the caller without first being freed to the
pool.  Therefore, a call to put_page_testzero() is in order so that such a
page will be handed to the caller with a correct count.

This has not affected end users because the bad page count is reset before the
page is handed off.  However, under CONFIG_DEBUG_VM this triggers a BUG when
the page count is validated.

Thanks go to Mel for first spotting this issue and providing an initial fix.
Signed-off-by: Adam Litke <agl@us.ibm.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Dave Hansen <haveblue@us.ibm.com>
Cc: William Lee Irwin III <wli@holomorphy.com>
Cc: Andy Whitcroft <apw@shadowen.org>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/hugetlb.c

@@ -286,6 +286,12 @@ static struct page *alloc_buddy_huge_page(struct vm_area_struct *vma,
 
 	spin_lock(&hugetlb_lock);
 	if (page) {
+		/*
+		 * This page is now managed by the hugetlb allocator and has
+		 * no users -- drop the buddy allocator's reference.
+		 */
+		put_page_testzero(page);
+		VM_BUG_ON(page_count(page));
 		nid = page_to_nid(page);
 		set_compound_page_dtor(page, free_huge_page);
 		/*
@@ -369,13 +375,14 @@ static int gather_surplus_pages(int delta)
 			enqueue_huge_page(page);
 		else {
 			/*
-			 * Decrement the refcount and free the page using its
-			 * destructor.  This must be done with hugetlb_lock
+			 * The page has a reference count of zero already, so
+			 * call free_huge_page directly instead of using
+			 * put_page.  This must be done with hugetlb_lock
 			 * unlocked which is safe because free_huge_page takes
 			 * hugetlb_lock before deciding how to free the page.
 			 */
 			spin_unlock(&hugetlb_lock);
-			put_page(page);
+			free_huge_page(page);
 			spin_lock(&hugetlb_lock);
 		}
 	}