Re: [PATCH 1/4] mm/migrate: Add migrate_device_split_page

[Zi Yan <ziy@nvidia.com>]

+THP folks

On 16 Dec 2025, at 15:10, Francois Dugast wrote:

> From: Matthew Brost <matthew.brost@intel.com>
>
> Introduce migrate_device_split_page() to split a device page into
> lower-order pages. Used when a folio allocated as higher-order is freed
> and later reallocated at a smaller order by the driver memory manager.
>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Balbir Singh <balbirs@nvidia.com>
> Cc: dri-devel@lists.freedesktop.org
> Cc: linux-mm@kvack.org
> Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> Signed-off-by: Francois Dugast <francois.dugast@intel.com>
> ---
>  include/linux/huge_mm.h |  3 +++
>  include/linux/migrate.h |  1 +
>  mm/huge_memory.c        |  6 ++---
>  mm/migrate_device.c     | 49 +++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 56 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index a4d9f964dfde..6ad8f359bc0d 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -374,6 +374,9 @@ int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list
>  int folio_split_unmapped(struct folio *folio, unsigned int new_order);
>  unsigned int min_order_for_split(struct folio *folio);
>  int split_folio_to_list(struct folio *folio, struct list_head *list);
> +int __split_unmapped_folio(struct folio *folio, int new_order,
> +			   struct page *split_at, struct xa_state *xas,
> +			   struct address_space *mapping, enum split_type split_type);
>  int folio_check_splittable(struct folio *folio, unsigned int new_order,
>  			   enum split_type split_type);
>  int folio_split(struct folio *folio, unsigned int new_order, struct page *page,
> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> index 26ca00c325d9..ec65e4fd5f88 100644
> --- a/include/linux/migrate.h
> +++ b/include/linux/migrate.h
> @@ -192,6 +192,7 @@ void migrate_device_pages(unsigned long *src_pfns, unsigned long *dst_pfns,
>  			unsigned long npages);
>  void migrate_device_finalize(unsigned long *src_pfns,
>  			unsigned long *dst_pfns, unsigned long npages);
> +int migrate_device_split_page(struct page *page);
>
>  #endif /* CONFIG_MIGRATION */
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 40cf59301c21..7ded35a3ecec 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3621,9 +3621,9 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
>   * Return: 0 - successful, <0 - failed (if -ENOMEM is returned, @folio might be
>   * split but not to @new_order, the caller needs to check)
>   */
> -static int __split_unmapped_folio(struct folio *folio, int new_order,
> -		struct page *split_at, struct xa_state *xas,
> -		struct address_space *mapping, enum split_type split_type)
> +int __split_unmapped_folio(struct folio *folio, int new_order,
> +			   struct page *split_at, struct xa_state *xas,
> +			   struct address_space *mapping, enum split_type split_type)
>  {
>  	const bool is_anon = folio_test_anon(folio);
>  	int old_order = folio_order(folio);
> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
> index 23379663b1e1..eb0f0e938947 100644
> --- a/mm/migrate_device.c
> +++ b/mm/migrate_device.c
> @@ -775,6 +775,49 @@ int migrate_vma_setup(struct migrate_vma *args)
>  EXPORT_SYMBOL(migrate_vma_setup);
>
>  #ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
> +/**
> + * migrate_device_split_page() - Split device page
> + * @page: Device page to split
> + *
> + * Splits a device page into smaller pages. Typically called when reallocating a
> + * folio to a smaller size. Inherently racy—only safe if the caller ensures
> + * mutual exclusion within the page's folio (i.e., no other threads are using
> + * pages within the folio). Expected to be called a free device page and
> + * restores all split out pages to a free state.
> + */
> +int migrate_device_split_page(struct page *page)
> +{
> +	struct folio *folio = page_folio(page);
> +	struct dev_pagemap *pgmap = folio->pgmap;
> +	struct page *unlock_page = folio_page(folio, 0);
> +	unsigned int order = folio_order(folio), i;
> +	int ret = 0;
> +
> +	VM_BUG_ON_FOLIO(!order, folio);
> +	VM_BUG_ON_FOLIO(!folio_is_device_private(folio), folio);
> +	VM_BUG_ON_FOLIO(folio_ref_count(folio), folio);
> +
> +	folio_lock(folio);
> +
> +	ret = __split_unmapped_folio(folio, 0, page, NULL, NULL, SPLIT_TYPE_UNIFORM);
> +	if (ret) {
> +	       /*
> +		* We can't fail here unless the caller doesn't know what they
> +		* are doing.
> +		*/
> +		VM_BUG_ON_FOLIO(ret, folio);
> +
> +		return ret;
> +	}
> +
> +	for (i = 0; i < 0x1 << order; ++i, ++unlock_page) {
> +		page_folio(unlock_page)->pgmap = pgmap;
> +		folio_unlock(page_folio(unlock_page));
> +	}
> +
> +	return 0;
> +}
> +
>  /**
>   * migrate_vma_insert_huge_pmd_page: Insert a huge folio into @migrate->vma->vm_mm
>   * at @addr. folio is already allocated as a part of the migration process with
> @@ -927,6 +970,11 @@ static int migrate_vma_split_unmapped_folio(struct migrate_vma *migrate,
>  	return ret;
>  }
>  #else /* !CONFIG_ARCH_ENABLE_THP_MIGRATION */
> +int migrate_device_split_page(struct page *page)
> +{
> +	return 0;
> +}
> +
>  static int migrate_vma_insert_huge_pmd_page(struct migrate_vma *migrate,
>  					 unsigned long addr,
>  					 struct page *page,
> @@ -943,6 +991,7 @@ static int migrate_vma_split_unmapped_folio(struct migrate_vma *migrate,
>  	return 0;
>  }
>  #endif
> +EXPORT_SYMBOL(migrate_device_split_page);
>
>  static unsigned long migrate_vma_nr_pages(unsigned long *src)
>  {
> -- 
> 2.43.0


Best Regards,
Yan, Zi