Re: [PATCH] mm: migrate: transfer large_rmappable flag in folio_migrate_flags()

[David Hildenbrand (Arm)]

On 3/11/26 14:23, Usama Arif wrote:
> folio_migrate_flags() transfers folio state from source to destination
> during migration, but does not transfer the large_rmappable flag.
> 
> Migration allocators like alloc_migration_target() and
> alloc_misplaced_dst_folio() use __folio_alloc() directly without
> wrapping the result in page_rmappable_folio(), so the destination folio
> never gets large_rmappable set.
> 
> This becomes a problem when a folio on the deferred split queue is
> migrated: the destination folio can be added to the deferred split queue
> via deferred_split_folio() (which does not check large_rmappable), but
> when the folio is later freed, folio_unqueue_deferred_split() bails out
> early because large_rmappable is not set:
> 
>   if (folio_order(folio) <= 1 || !folio_test_large_rmappable(folio))
>       return false;
> 
> This leaves a stale entry on the deferred split queue, leading to
> use-after-free when the shrinker walks the list.
> 
> Fix this by transferring large_rmappable in folio_migrate_flags(),
> consistent with how all other folio flags are handled.
> 
> Fixes: dafff3f4c850 ("mm: split underused THPs")
> Signed-off-by: Usama Arif <usama.arif@xxxxxxxxx>
> ---
>  mm/migrate.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 3380021fd3db..ee1c7bc851dd 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -846,6 +846,9 @@ void folio_migrate_flags(struct folio *newfolio, struct folio *folio)
>  	folio_copy_owner(newfolio, folio);
>  	pgalloc_tag_swap(newfolio, folio);
>  
> +	if (folio_test_large_rmappable(folio))
> +		folio_set_large_rmappable(newfolio);
> +
>  	mem_cgroup_migrate(folio, newfolio);
>  }
>  EXPORT_SYMBOL(folio_migrate_flags);

compaction_alloc_noprof() does the page_rmappable_folio() at the end.

I'd assume that all migration allocation functions must take care of that.

It's a responsibility of the folio allocation code, not folio migration
code.


-- 
Cheers,

David