Re: [PATCH 1/3] mm/huge_memory: add folio_split_driver_managed()

[Zi Yan]

On Wed Jul 22, 2026 at 12:42 AM EDT, Matthew Brost wrote:
> Add a lightweight structural split primitive for large (compound) folios
> that a driver allocated with __GFP_COMP and manages entirely by itself,
> outside of the core mm's view.
>
> The existing split paths - split_folio() and folio_split_unmapped() -
> are built for folios that the mm owns: they perform a refcount freeze,
> walk and remap the rmap, and take the anon_vma / i_mmap locks, and
> folio_split_unmapped() further assumes an anon, pagecache-style refcount
> model (nr_pages + 1). None of that applies to a folio that is:
>
>   - singly referenced (the caller holds the only reference),
>   - not mapped through the rmap (folio_mapped() == 0),
>   - not in the page cache or swap cache (folio->mapping == NULL),
>   - not on any LRU or the deferred-split list.
>
> For such a folio the split is purely structural: because nothing else in
> the kernel can reach it, there is no need to freeze the refcount or touch
> any mapping. folio_split_driver_managed() therefore performs only the

I do not think so. PFN scanners like memory compaction should be able to
see them, unless you mean something else about "a driver allocated with
__GFP_COMP". You will need to freeze it to prevent the to-be-split
folios being touched by others.


> compound and split-accounting teardown via __split_unmapped_folio() and
> then hands each resulting order-@new_order folio its own reference,
> mirroring split_page() for compound folios. The caller keeps the original
> reference on the first resulting folio and is responsible for freeing all
> of them individually.
>
> The immediate user is TTM's GPU page pool, which allocates higher-order
> compound pages, maps them into userspace via VM_PFNMAP (never through the
> rmap), and needs to split them into order-0 folios under memory pressure
> so pages can be backed up to shmem and freed one at a time.
>
> A CONFIG_TRANSPARENT_HUGEPAGE=n stub is provided so callers can build
> without the split machinery; it warns and returns -EINVAL.
>
> Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
> Cc: Maxime Ripard <mripard@xxxxxxxxxx>
> Cc: Thomas Zimmermann <tzimmermann@xxxxxxx>
> Cc: David Airlie <airlied@xxxxxxxxx>
> Cc: Simona Vetter <simona@xxxxxxxx>
> Cc: Christian Koenig <christian.koenig@xxxxxxx>
> Cc: Huang Rui <ray.huang@xxxxxxx>
> Cc: Matthew Auld <matthew.auld@xxxxxxxxx>
> Cc: Matthew Brost <matthew.brost@xxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Lorenzo Stoakes <ljs@xxxxxxxxxx>
> Cc: Zi Yan <ziy@xxxxxxxxxx>
> Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> Cc: "Liam R. Howlett" <liam@xxxxxxxxxxxxx>
> Cc: Nico Pache <npache@xxxxxxxxxx>
> Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
> Cc: Dev Jain <dev.jain@xxxxxxx>
> Cc: Barry Song <baohua@xxxxxxxxxx>
> Cc: Lance Yang <lance.yang@xxxxxxxxx>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxx>
> Cc: Dave Airlie <airlied@xxxxxxxxxx>
> Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: linux-mm@xxxxxxxxx
> Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Signed-off-by: Matthew Brost <matthew.brost@xxxxxxxxx>
> Assisted-by: GitHub-Copilot:claude-opus-4.8
>
> ---
>
> The patch is based on drm-tip rather than the core MM branches to
> facilitate Intel CI testing and initial review. It can be rebased onto
> the core MM branches in a subsequent revision.
> ---
>  include/linux/huge_mm.h |  8 ++++++
>  mm/huge_memory.c        | 63 +++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 71 insertions(+)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index ad20f7f8c179..35661d82d54a 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -402,6 +402,7 @@ enum split_type {
>  int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>  		unsigned int new_order);
>  int folio_split_unmapped(struct folio *folio, unsigned int new_order);
> +int folio_split_driver_managed(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 folio_check_splittable(struct folio *folio, unsigned int new_order,
> @@ -656,6 +657,13 @@ static inline int split_folio_to_list(struct folio *folio, struct list_head *lis
>  	return -EINVAL;
>  }
>  
> +static inline int folio_split_driver_managed(struct folio *folio,
> +					     unsigned int new_order)
> +{
> +	VM_WARN_ON_ONCE_FOLIO(1, folio);
> +	return -EINVAL;
> +}
> +
>  static inline int folio_split(struct folio *folio, unsigned int new_order,
>  		struct page *page, struct list_head *list)
>  {
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 2bccb0a53a0a..06f9a5f35df8 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -4185,6 +4185,69 @@ int folio_split_unmapped(struct folio *folio, unsigned int new_order)
>  	return ret;
>  }
>  
> +/**
> + * folio_split_driver_managed() - split an exclusively-owned, off-LRU folio
> + * @folio: folio to split. Must be a large (compound) folio that is owned
> + *         exclusively by the caller and is invisible to the core mm.
> + * @new_order: the order of the folios after the split.
> + *
> + * This is a lightweight structural split for folios that a driver allocated
> + * and manages itself (for example TTM's GPU page pool, which allocates
> + * higher-order compound pages with __GFP_COMP and maps them into userspace
> + * via VM_PFNMAP rather than through the rmap). Such folios are:
> + *
> + *   - singly referenced (the caller holds the only reference),
> + *   - not mapped through the rmap (folio_mapcount() == 0),
> + *   - not in the page cache or swap cache (folio->mapping == NULL),
> + *   - not on any LRU or the deferred-split list.
> + *
> + * Because nothing else in the kernel can reach the folio, this helper does
> + * not perform the refcount freeze / remap / anon_vma & i_mmap locking dance
> + * that split_folio() and folio_split_unmapped() require. It performs only
> + * the compound and split-accounting teardown and then hands each resulting
> + * folio its own reference, mirroring split_page() for compound folios.
> + *
> + * The caller is responsible for freeing the resulting folios individually.
> + *
> + * Context: caller holds the only reference and excludes concurrent access.
> + * Does not sleep.
> + *
> + * Return: 0 on success, negative errno on failure.
> + */
> +int folio_split_driver_managed(struct folio *folio, unsigned int new_order)
> +{
> +	unsigned int old_order = folio_order(folio);
> +	unsigned int split_nr = 1U << new_order;
> +	unsigned int nr = 1U << old_order;
> +	unsigned int i;
> +
> +	if (new_order >= old_order)
> +		return -EINVAL;
> +
> +	VM_WARN_ON_ONCE_FOLIO(folio_ref_count(folio) != 1, folio);
> +	VM_WARN_ON_ONCE_FOLIO(folio_mapped(folio), folio);
> +	VM_WARN_ON_ONCE_FOLIO(folio->mapping, folio);
> +	VM_WARN_ON_ONCE_FOLIO(folio_test_swapcache(folio), folio);
> +	VM_WARN_ON_ONCE_FOLIO(folio_test_lru(folio), folio);
> +
> +	/*
> +	 * Structural + split-accounting teardown only. No mapping/xarray, no
> +	 * refcount freeze: the folio is frozen-by-ownership already.
> +	 */
> +	__split_unmapped_folio(folio, new_order, &folio->page, NULL, NULL,
> +			       SPLIT_TYPE_UNIFORM);
> +
> +	/*
> +	 * Give every resulting head folio its own reference. The original
> +	 * reference stays on the first one, exactly like split_page().
> +	 */
> +	for (i = split_nr; i < nr; i += split_nr)
> +		set_page_refcounted(folio_page(folio, i));
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(folio_split_driver_managed);
> +
>  /*
>   * This function splits a large folio into smaller folios of order @new_order.
>   * @page can point to any page of the large folio to split. The split operation




-- 
Best Regards,
Yan, Zi