[PATCH v3 3/5] mm/huge_memory: add page->private check back in __split_folio_to_order()

[Zi Yan]

page->private should not be set in tail pages. Commit 4265d67e405a
("mm/migrate_device: add THP splitting during migration") removed a
DEBUG_VM check without a proper reason[1]. Add it back.

Link: https://lore.kernel.org/all/13f3fcda-7328-4aa5-afc6-75a294a82b2a@xxxxxxxxxx/ [1]
Reviewed-by: Lance Yang <lance.yang@xxxxxxxxx>
Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
Reviewed-by: Vlastimil Babka (SUSE) <vbabka@xxxxxxxxxx>
Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
---
 mm/huge_memory.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index fb2d7dd17de0c..ea0445d6c17f0 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3591,6 +3591,13 @@ 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. Warn once
+		 * if private is unexpectedly set. Do it before swap.val assignment
+		 * since private overlaps with swap.val.
+		 */
+		VM_WARN_ON_ONCE_PAGE(new_folio->private, new_head);
+
 		if (folio_test_swapcache(folio))
 			new_folio->swap.val = folio->swap.val + i;
 

-- 
2.53.0