Re: [PATCH v4] mm/migrate_device: Clear stale mapping after freeing swapcache

["David Hildenbrand (Arm)" <david@kernel.org>]

On 7/28/26 08:28, 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 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@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 | 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;

This function is surely a beauty. (had to rephrase that sentence 3 times ;) )

migrate_vma_split_unmapped_folio() modifies the src_pfns() entries on success.
(and somehow assumes that it's always a THP, what? After a MIGRATE_PFN_COMPOUND
value is set? What? Why the "nr = 1 << folio_order(folio);" in the caller).

If we ended up modifying the current entry in such a way, shouldn't we just have
retry: label and restart at the very top of the function, where we just
naturally re-read the entry/page/folio and do the right thing?

>  			} 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;

While this looks good, I do wonder why do we have to supply the mapping here at all?

Is there a path where we call folio_migrate_mapping() and the old folio (folio)
does no longer have the right mapping attached?

It would be a lot less error prone if the function would just obtain the mapping
from the old folio.

-- 
Cheers,

David