[PATCH v2] mm/migrate_device: Clear stale mapping after freeing swapcache

[Arvind Yadav]

__migrate_device_pages() reads the folio mapping before calling
folio_free_swap(). When folio_free_swap() succeeds, the folio is removed
from the swap cache, but the saved mapping still points to swap_space.

Passing the stale mapping to folio_migrate_mapping() makes it take the
mapped-folio path after the swapcache reference has been dropped. This can
cause an invalid swap_space lock access followed by a folio reference
count BUG.

Refresh the saved mapping after folio_free_swap() so the current folio
state is used during migration.

v2:
- Refresh the mapping using folio_mapping(), as suggested by Zi Yan.

Fixes: df263d9a7dff ("mm/migrate_device: try to handle swapcache pages")
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Matthew Brost <matthew.brost@xxxxxxxxx>
Cc: Joshua Hahn <joshua.hahnjy@xxxxxxxxx>
Cc: Zi Yan <ziy@xxxxxxxxxx>
Cc: Rakie Kim <rakie.kim@xxxxxx>
Cc: Byungchul Park <byungchul@xxxxxx>
Cc: Gregory Price <gourry@xxxxxxxxxx>
Cc: Ying Huang <ying.huang@xxxxxxxxxxxxxxxxx>
Cc: Alistair Popple <apopple@xxxxxxxxxx>
Signed-off-by: Arvind Yadav <arvind.yadav@xxxxxxxxx>
---
 mm/migrate_device.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index 554754eb26ff..ae895758b72c 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -1204,6 +1204,12 @@ static void __migrate_device_pages(unsigned long *src_pfns,
 					src_pfns[i] &= ~MIGRATE_PFN_MIGRATE;
 					goto next;
 				}
+
+				/*
+				 * folio_free_swap() removed the folio from the swap
+				 * cache. Refresh the saved mapping before migration.
+				 */
+				mapping = folio_mapping(folio);
 			}
 		} else if (folio_is_zone_device(newfolio)) {
 			/*
-- 
2.43.0