[PATCH v4] 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 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.

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

v3:
- Reset nr to 1 after a successful split so each resulting folio is
  processed independently, as suggested by Zi Yan.

v4:
- Re-read each source folio's mapping immediately before
  folio_migrate_mapping(), as suggested by Balbir Singh.
- Add a warning to validate the post-split order-0 invariant,
  as suggested by Balbir Singh.

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: 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>
Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>
Reviewed-by: Balbir Singh <balbirs@xxxxxxxxxx>
Signed-off-by: Arvind Yadav <arvind.yadav@xxxxxxxxx>
---
 mm/migrate_device.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index 554754eb26ff..0e6f00190fd8 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -1182,6 +1182,13 @@ static void __migrate_device_pages(unsigned long *src_pfns,
 							 MIGRATE_PFN_COMPOUND);
 					goto next;
 				}
+
+				/*
+				 * reset nr so that only first after-split folio
+				 * is processed below
+				 */
+				VM_WARN_ON_ONCE(folio_test_large(folio));
+				nr = 1;
 			} else if ((src_pfns[i] & MIGRATE_PFN_MIGRATE) &&
 				(dst_pfns[i] & MIGRATE_PFN_COMPOUND) &&
 				!(src_pfns[i] & MIGRATE_PFN_COMPOUND)) {
@@ -1221,6 +1228,12 @@ static void __migrate_device_pages(unsigned long *src_pfns,
 			folio = page_folio(migrate_pfn_to_page(src_pfns[i+j]));
 			newfolio = page_folio(migrate_pfn_to_page(dst_pfns[i+j]));
 
+			/*
+			 * folio_free_swap() removed the folio from the swap
+			 * cache. Refresh the saved mapping before migration.
+			 */
+			mapping = folio_mapping(folio);
+
 			r = folio_migrate_mapping(mapping, newfolio, folio, extra_cnt);
 			if (r)
 				src_pfns[i+j] &= ~MIGRATE_PFN_MIGRATE;
-- 
2.43.0