Do not lose the flag on page migration.

Ideally, these folios should be freed instead of migration. But it
requires to find right spot do this and proper testing.

Transfer the flag for now.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 mm/migrate.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/migrate.c b/mm/migrate.c
index caadbe393aa2..690efa064bee 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -682,6 +682,10 @@ void folio_migrate_flags(struct folio *newfolio, struct folio *folio)
 	if (folio_test_dirty(folio))
 		folio_set_dirty(newfolio);
 
+	/* TODO: free the folio on migration? */
+	if (folio_test_dropbehind(folio))
+		folio_set_dropbehind(newfolio);
+
 	if (folio_test_young(folio))
 		folio_set_young(newfolio);
 	if (folio_test_idle(folio))
-- 
2.45.2