mm/memory_hotplug: use pfn math in place of direct struct page manipulation

When dealing with hugetlb pages, manipulating struct page pointers
directly can get to wrong struct page, since struct page is not guaranteed
to be contiguous on SPARSEMEM without VMEMMAP.  Use pfn calculation to
handle it properly.

Without the fix, a wrong number of page might be skipped. Since skip cannot be
negative, scan_movable_page() will end early and might miss a movable page with
-ENOENT. This might fail offline_pages(). No bug is reported. The fix comes
from code inspection.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: eeb0efd ("mm,memory_hotplug: fix scan_movable_pages() for gigantic hugepages")
Signed-off-by: Zi Yan <[email protected]>
Reviewed-by: Muchun Song <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Mike Rapoport (IBM) <[email protected]>
Cc: Thomas Bogendoerfer <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: x-y-z

mm/memory_hotplug.c

@@ -1689,7 +1689,7 @@ static int scan_movable_pages(unsigned long start, unsigned long end,
 		 */
 		if (HPageMigratable(head))
 			goto found;
-		skip = compound_nr(head) - (page - head);
+		skip = compound_nr(head) - (pfn - page_to_pfn(head));
 		pfn += skip - 1;
 	}
 	return -ENOENT;