[PATCH 6.12.y] mm/migrate_device: clear stale mapping after freeing swapcache

[Sasha Levin <sashal@kernel.org>]

From: Arvind Yadav <arvind.yadav@intel.com>

[ Upstream commit 34a00895d032a414830d41106a09329ae6c251b6 ]

__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 use the
mapped-folio path for a folio that is no longer in swapcache.  It can then
operate on swap_space.i_pages with invalid reference accounting,
eventually triggering a folio reference count BUG.

After a successful split, nr still contains the number of pages in the
original large folio, although each resulting page is now a separate
order-0 folio.  Reset nr to 1 so each split folio is processed separately,
including its own swapcache removal and mapping lookup.

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

Link: https://lore.kernel.org/20260728062832.1107127-1-arvind.yadav@intel.com
Fixes: df263d9a7dff ("mm/migrate_device: try to handle swapcache pages")
Signed-off-by: Arvind Yadav <arvind.yadav@intel.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Balbir Singh <balbirs@nvidia.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Gregory Price <gourry@gourry.net>
Cc: Ying Huang <ying.huang@linux.alibaba.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
[ refreshed folio_mapping() after folio_free_swap() instead of resetting the not-yet-present THP-split nr/batching loop ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 mm/migrate_device.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index 94e0b33bd99cb..d655f3c0f3e76 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -746,6 +746,12 @@ static void __migrate_device_pages(unsigned long *src_pfns,
 					src_pfns[i] &= ~MIGRATE_PFN_MIGRATE;
 					continue;
 				}
+
+				/*
+				 * 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)) {
 			/*
-- 
2.53.0