page->private should not be set in tail pages. Commit 4265d67e405a
("mm/migrate_device: add THP splitting during migration") removed it
without a proper reason. Add it back.

Signed-off-by: Zi Yan <ziy@nvidia.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
Cc: Nico Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: linux-mm@kvack.org
---
 mm/huge_memory.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 27316d741927..6b57d219fc97 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3548,6 +3548,16 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
 		new_folio->mapping = folio->mapping;
 		new_folio->index = folio->index + i;
 
+		/*
+		 * page->private should not be set in tail pages. Fix up and warn once
+		 * if private is unexpectedly set. Do it before swap.val assignment
+		 * since private overlaps with swap.val.
+		 */
+		if (unlikely(new_folio->private)) {
+			VM_WARN_ON_ONCE_PAGE(true, new_head);
+			new_folio->private = NULL;
+		}
+
 		if (folio_test_swapcache(folio))
 			new_folio->swap.val = folio->swap.val + i;
 
-- 
2.51.0