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

[Zi Yan]

On Sat Jul 25, 2026 at 3:36 PM EDT, David Hildenbrand (Arm) wrote:
> On 7/25/26 06:43, Andrew Morton wrote:
>> On Fri, 24 Jul 2026 13:57:02 +0530 Arvind Yadav <arvind.yadav@xxxxxxxxx> wrote:
>> 
>>> __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.
>>>
>> 
>> Thanks.  AI review might have found an issue with this.  And one
>> possible pre-existing issue in the code which Alistair and Balbir
>> worked on.
>> 
>> 	https://sashiko.dev/#/patchset/20260724082702.2531024-1-arvind.yadav@xxxxxxxxx
>
> Yeah, this might need another careful look.

It seems that the pre-existing issue can be fixed by resetting nr to 1
after split is successful. It should also complete this patch. Something
like this:


diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index 18d097c388530..4a77b6c86ae4f 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -1193,6 +1193,11 @@ 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
+				 */
+				nr = 1;
 			} else if ((src_pfns[i] & MIGRATE_PFN_MIGRATE) &&
 				(dst_pfns[i] & MIGRATE_PFN_COMPOUND) &&
 				!(src_pfns[i] & MIGRATE_PFN_COMPOUND)) {


-- 
Best Regards,
Yan, Zi