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

[Balbir Singh]

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@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 | 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))

>  			} 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);



>  		} else if (folio_is_zone_device(newfolio)) {
>  			/*

Thanks,
Balbir