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

["Yadav, Arvind" <arvind.yadav@intel.com>]

On 24-07-2026 01:09, Zi Yan wrote:
> On 23 Jul 2026, at 6:08, Arvind Yadav 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.
>>
>> Clear the saved mapping so the source is migrated as a mappingless
>> anonymous folio.
> Why not use folio_mapping(folio) in the call to folio_migrate_mapping()?
> Or do mapping = folio_mapping(folio) instead of a hard-coded mapping = NULL?


Agree, Re-reading the mapping is clearer and avoids assuming it is 
always NULL. I’ll update it to refresh mapping after folio_free_swap().
Thanks for the suggestion.

Regards,
Arvind

>
>> Fixes: df263d9a7dff ("mm/migrate_device: try to handle swapcache pages")
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: David Hildenbrand <david@kernel.org>
>> Cc: Matthew Brost <matthew.brost@intel.com>
>> Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
>> Cc: Zi Yan <ziy@nvidia.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>
>> Signed-off-by: Arvind Yadav <arvind.yadav@intel.com>
>> ---
>>   mm/migrate_device.c | 6 ++++++
>>   1 file changed, 6 insertions(+)
>>
>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
>> index 554754eb26ff..fa7fbacac514 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. Migrate it as a mappingless anonymous folio.
>> +				 */
>> +				mapping = NULL;
>>   			}
>>   		} else if (folio_is_zone_device(newfolio)) {
>>   			/*
>> -- 
>> 2.43.0
>
> Best Regards,
> Yan, Zi