hugetlb: make unmap_ref_private multi-size-aware

Oops.  Part of the hugetlb private reservation code was not fully
converted to use hstates.

When a huge page must be unmapped from VMAs due to a failed COW,
HPAGE_SIZE is used in the call to unmap_hugepage_range() regardless of
the page size being used.  This works if the VMA is using the default
huge page size.  Otherwise we might unmap too much, too little, or
trigger a BUG_ON.  Rare but serious -- fix it.
Signed-off-by: Adam Litke <agl@us.ibm.com>
Cc: Jon Tollefson <kniht@linux.vnet.ibm.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/hugetlb.c

@@ -1796,6 +1796,7 @@ void unmap_hugepage_range(struct vm_area_struct *vma, unsigned long start,
 static int unmap_ref_private(struct mm_struct *mm, struct vm_area_struct *vma,
 				struct page *page, unsigned long address)
 {
+	struct hstate *h = hstate_vma(vma);
 	struct vm_area_struct *iter_vma;
 	struct address_space *mapping;
 	struct prio_tree_iter iter;
@@ -1805,7 +1806,7 @@ static int unmap_ref_private(struct mm_struct *mm, struct vm_area_struct *vma,
 	 * vm_pgoff is in PAGE_SIZE units, hence the different calculation
 	 * from page cache lookup which is in HPAGE_SIZE units.
 	 */
-	address = address & huge_page_mask(hstate_vma(vma));
+	address = address & huge_page_mask(h);
 	pgoff = ((address - vma->vm_start) >> PAGE_SHIFT)
 		+ (vma->vm_pgoff >> PAGE_SHIFT);
 	mapping = (struct address_space *)page_private(page);
@@ -1824,7 +1825,7 @@ static int unmap_ref_private(struct mm_struct *mm, struct vm_area_struct *vma,
 		 */
 		if (!is_vma_resv_set(iter_vma, HPAGE_RESV_OWNER))
 			unmap_hugepage_range(iter_vma,
-				address, address + HPAGE_SIZE,
+				address, address + huge_page_size(h),
 				page);
 	}