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

[Matthew Brost]

On Mon, Jul 27, 2026 at 02:23:33PM -0400, Zi Yan wrote:
> On 27 Jul 2026, at 13:33, David Hildenbrand (Arm) wrote:
> 
> > On 7/22/26 17:28, Zi Yan wrote:
> >> On Wed Jul 22, 2026 at 10:26 AM EDT, Zi Yan wrote:
> >>> 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:
> >>
> >> Strickly speaking, these vm_insert*() compound pages are not folios,
> >> since folios are supposed to be rmappable and they are either anonymous
> >> memory or file-backed memory. I am working on separating them from
> >> rmappable folios by replacing PG_private with PG_folio and marking all
> >> pages in a folio with PG_folio in page_rmappable_folio().
> >>
> >> Hopefully, we can find a better name, like refcounted_folio, later for
> >> these non-rmappable compound pages.
> >
> > They wouldn't really be folios, I guess. They would likely be a simple
> > "refcounted" memtype that allows for compound pages.
> 
> Yes, they are not folios. But “folio” was started to replace “compound page”
> and slowly becomes rmappable anon and file-backed. People outside MM still
> thinks “folio” == “compound page”. But once I manage to remove PG_private
> and get us PG_folio, page_folio() will return NULL for non-folio compound
> pages and we will need a new type for them, “refcounted_XXX”. We can decide
> XXX when I get there. :)
> 
> >
> > But what is the conclusion here? It sounds like "folio_split_" is the entirely
> > wrong interface for these compound pages.
> 
> We probably would allow folio_split() to be used on compound pages now
> until we can make a clean distinction, e.g., using PG_folio, between them.
> 
> Yes, folio_split() and its helper functions are meant for rmappable anon and
> file-backed folios. But currently “folio” is de facto “compound page”, since
> for example prep_compound_head() initializes folio fields even if it is meant
> only for compound pages.
> 
> After folio and compound page are separate concepts in the code base, probably
> we can think about how to have two split functions for them and still keep
> maximum code reuse. Namely, we could have split_compound() does the compound
> page split (e.g., copying page flags, adjust compound head/tail/order, etc.)
> and split_folio() calls split_compound() and perform extra folio operations.
> The details are TBD.

I like the idea of split_compound, ideally in mm/page_alloc.c, similar
to split_page(), which performs the mechanical splitting of compound
pages managed on the driver side (i.e., not mappable, not on an LRU,
protected by dma-resv in the case of DRM, etc.).

I got yelled at for not allocating these types of pages with GFP_COMP,
and I basically found that this missing MM primitive is what has
prevented that approach from being practical. Using GFP_COMP makes our
life quite a bit easier for managing these types of pages too, so it
would be great if we could get a solution for this.

So maybe I should drop this for now, keep an eye on the MM work, and
circle back once the missing primitive is implemented?

Thanks the help / insight,
Matt

> 
> Best Regards,
> Yan, Zi