Re: [PATCH v3] mm/migrate_device: Clear stale mapping after freeing swapcache

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

On 27-07-2026 13:30, Balbir Singh wrote:
> On 7/27/26 3:39 PM, 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.
>>
>> 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.
>>
>> 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: 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>
>> Reviewed-by: Zi Yan <ziy@nvidia.com>
>> Reviewed-by: Balbir Singh <balbirs@nvidia.com>
>> Signed-off-by: Arvind Yadav <arvind.yadav@intel.com>
>> ---
>>   mm/migrate_device.c | 11 +++++++++++
>>   1 file changed, 11 insertions(+)
>>
>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
>> index 554754eb26ff..74ac6651d4b3 100644
>> --- a/mm/migrate_device.c
>> +++ b/mm/migrate_device.c
>> @@ -1182,6 +1182,11 @@ 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;
> We should also add a VM_WARN_ON_ONCE(folio_test_swapcache(folio))


Agreed , but folio_test_swapcache()  is not valid immediately after the 
split, as the order-0 folios may still be in swapcache.
I’ll use just before nr = 1

>
>>   			} else if ((src_pfns[i] & MIGRATE_PFN_MIGRATE) &&
>>   				(dst_pfns[i] & MIGRATE_PFN_COMPOUND) &&
>>   				!(src_pfns[i] & MIGRATE_PFN_COMPOUND)) {
>> @@ -1204,6 +1209,12 @@ 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);
>>   			}
> I wonder if we should change this instead to (sorry for the wrap around, I'll
> need to fix my mailer)
>
> @@ -1242,6 +1232,11 @@ static void __migrate_device_pages(unsigned long *src_pfns,
>                  for (j = 0; j < nr && i + j < npages; j++) {
>                          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);
>

Agreed, Moving the mapping lookup into the per-folio loop looks better; 
I’ll update it


Thanks,
Arvind

>
>>   		} else if (folio_is_zone_device(newfolio)) {
>>   			/*
> Thanks,
> Balbir