Re: [PATCH v2 11/11] mm: Remove device private pages from the physical address space

[Zi Yan <ziy@nvidia.com>]

On 7 Jan 2026, at 4:18, Jordan Niethe wrote:

> Currently when creating these device private struct pages, the first
> step is to use request_free_mem_region() to get a range of physical
> address space large enough to represent the devices memory. This
> allocated physical address range is then remapped as device private
> memory using memremap_pages().
>
> Needing allocation of physical address space has some problems:
>
>   1) There may be insufficient physical address space to represent the
>      device memory. KASLR reducing the physical address space and VM
>      configurations with limited physical address space increase the
>      likelihood of hitting this especially as device memory increases. This
>      has been observed to prevent device private from being initialized.
>
>   2) Attempting to add the device private pages to the linear map at
>      addresses beyond the actual physical memory causes issues on
>      architectures like aarch64 meaning the feature does not work there.
>
> Instead of using the physical address space, introduce a device private
> address space and allocate devices regions from there to represent the
> device private pages.
>
> Introduce a new interface memremap_device_private_pagemap() that
> allocates a requested amount of device private address space and creates
> the necessary device private pages.
>
> To support this new interface, struct dev_pagemap needs some changes:
>
>   - Add a new dev_pagemap::nr_pages field as an input parameter.
>   - Add a new dev_pagemap::pages array to store the device
>     private pages.
>
> When using memremap_device_private_pagemap(), rather then passing in
> dev_pagemap::ranges[dev_pagemap::nr_ranges] of physical address space to
> be remapped, dev_pagemap::nr_ranges will always be 1, and the device
> private range that is reserved is returned in dev_pagemap::range.
>
> Forbid calling memremap_pages() with dev_pagemap::ranges::type =
> MEMORY_DEVICE_PRIVATE.
>
> Represent this device private address space using a new
> device_private_pgmap_tree maple tree. This tree maps a given device
> private address to a struct dev_pagemap, where a specific device private
> page may then be looked up in that dev_pagemap::pages array.
>
> Device private address space can be reclaimed and the assoicated device
> private pages freed using the corresponding new
> memunmap_device_private_pagemap() interface.
>
> Because the device private pages now live outside the physical address
> space, they no longer have a normal PFN. This means that page_to_pfn(),
> et al. are no longer meaningful.
>
> Introduce helpers:
>
>   - device_private_page_to_offset()
>   - device_private_folio_to_offset()
>
> to take a given device private page / folio and return its offset within
> the device private address space.
>
> Update the places where we previously converted a device private page to
> a PFN to use these new helpers. When we encounter a device private
> offset, instead of looking up its page within the pagemap use
> device_private_offset_to_page() instead.
>
> Update the existing users:
>
>  - lib/test_hmm.c
>  - ppc ultravisor
>  - drm/amd/amdkfd
>  - gpu/drm/xe
>  - gpu/drm/nouveau
>
> to use the new memremap_device_private_pagemap() interface.
>
> Signed-off-by: Jordan Niethe <jniethe@nvidia.com>
> Signed-off-by: Alistair Popple <apopple@nvidia.com>
>
> ---
>
> NOTE: The updates to the existing drivers have only been compile tested.
> I'll need some help in testing these drivers.
>
> v1:
> - Include NUMA node paramater for memremap_device_private_pagemap()
> - Add devm_memremap_device_private_pagemap() and friends
> - Update existing users of memremap_pages():
>     - ppc ultravisor
>     - drm/amd/amdkfd
>     - gpu/drm/xe
>     - gpu/drm/nouveau
> - Update for HMM huge page support
> - Guard device_private_offset_to_page and friends with CONFIG_ZONE_DEVICE
>
> v2:
> - Make sure last member of struct dev_pagemap remains DECLARE_FLEX_ARRAY(struct range, ranges);
> ---
>  Documentation/mm/hmm.rst                 |  11 +-
>  arch/powerpc/kvm/book3s_hv_uvmem.c       |  41 ++---
>  drivers/gpu/drm/amd/amdkfd/kfd_migrate.c |  23 +--
>  drivers/gpu/drm/nouveau/nouveau_dmem.c   |  35 ++--
>  drivers/gpu/drm/xe/xe_svm.c              |  28 +---
>  include/linux/hmm.h                      |   3 +
>  include/linux/leafops.h                  |  16 +-
>  include/linux/memremap.h                 |  64 +++++++-
>  include/linux/migrate.h                  |   6 +-
>  include/linux/mm.h                       |   2 +
>  include/linux/rmap.h                     |   5 +-
>  include/linux/swapops.h                  |  10 +-
>  lib/test_hmm.c                           |  69 ++++----
>  mm/debug.c                               |   9 +-
>  mm/memremap.c                            | 193 ++++++++++++++++++-----
>  mm/mm_init.c                             |   8 +-
>  mm/page_vma_mapped.c                     |  19 ++-
>  mm/rmap.c                                |  43 +++--
>  mm/util.c                                |   5 +-
>  19 files changed, 391 insertions(+), 199 deletions(-)
>
<snip>

> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index e65329e1969f..b36599ab41ba 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2038,6 +2038,8 @@ static inline unsigned long memdesc_section(memdesc_flags_t mdf)
>   */
>  static inline unsigned long folio_pfn(const struct folio *folio)
>  {
> +	VM_BUG_ON(folio_is_device_private(folio));

Please use VM_WARN_ON instead.

> +
>  	return page_to_pfn(&folio->page);
>  }
>
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index 57c63b6a8f65..c1561a92864f 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -951,7 +951,7 @@ static inline unsigned long page_vma_walk_pfn(unsigned long pfn)
>  static inline unsigned long folio_page_vma_walk_pfn(const struct folio *folio)
>  {
>  	if (folio_is_device_private(folio))
> -		return page_vma_walk_pfn(folio_pfn(folio)) |
> +		return page_vma_walk_pfn(device_private_folio_to_offset(folio)) |
>  		       PVMW_PFN_DEVICE_PRIVATE;
>
>  	return page_vma_walk_pfn(folio_pfn(folio));
> @@ -959,6 +959,9 @@ static inline unsigned long folio_page_vma_walk_pfn(const struct folio *folio)
>
>  static inline struct page *page_vma_walk_pfn_to_page(unsigned long pvmw_pfn)
>  {
> +	if (pvmw_pfn & PVMW_PFN_DEVICE_PRIVATE)
> +		return device_private_offset_to_page(pvmw_pfn >> PVMW_PFN_SHIFT);
> +
>  	return pfn_to_page(pvmw_pfn >> PVMW_PFN_SHIFT);
>  }

<snip>

> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> index 96c525785d78..141fe5abd33f 100644
> --- a/mm/page_vma_mapped.c
> +++ b/mm/page_vma_mapped.c
> @@ -107,6 +107,7 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, pmd_t *pmdvalp,
>  static bool check_pte(struct page_vma_mapped_walk *pvmw, unsigned long pte_nr)
>  {
>  	unsigned long pfn;
> +	bool device_private = false;
>  	pte_t ptent = ptep_get(pvmw->pte);
>
>  	if (pvmw->flags & PVMW_MIGRATION) {
> @@ -115,6 +116,9 @@ static bool check_pte(struct page_vma_mapped_walk *pvmw, unsigned long pte_nr)
>  		if (!softleaf_is_migration(entry))
>  			return false;
>
> +		if (softleaf_is_migration_device_private(entry))
> +			device_private = true;
> +
>  		pfn = softleaf_to_pfn(entry);
>  	} else if (pte_present(ptent)) {
>  		pfn = pte_pfn(ptent);
> @@ -127,8 +131,14 @@ static bool check_pte(struct page_vma_mapped_walk *pvmw, unsigned long pte_nr)
>  			return false;
>
>  		pfn = softleaf_to_pfn(entry);
> +
> +		if (softleaf_is_device_private(entry))
> +			device_private = true;
>  	}
>
> +	if ((device_private) ^ !!(pvmw->pfn & PVMW_PFN_DEVICE_PRIVATE))
> +		return false;
> +
>  	if ((pfn + pte_nr - 1) < (pvmw->pfn >> PVMW_PFN_SHIFT))
>  		return false;
>  	if (pfn > ((pvmw->pfn >> PVMW_PFN_SHIFT) + pvmw->nr_pages - 1))
> @@ -137,8 +147,11 @@ static bool check_pte(struct page_vma_mapped_walk *pvmw, unsigned long pte_nr)
>  }
>
>  /* Returns true if the two ranges overlap.  Careful to not overflow. */
> -static bool check_pmd(unsigned long pfn, struct page_vma_mapped_walk *pvmw)
> +static bool check_pmd(unsigned long pfn, bool device_private, struct page_vma_mapped_walk *pvmw)
>  {
> +	if ((device_private) ^ !!(pvmw->pfn & PVMW_PFN_DEVICE_PRIVATE))
> +		return false;
> +
>  	if ((pfn + HPAGE_PMD_NR - 1) < (pvmw->pfn >> PVMW_PFN_SHIFT))
>  		return false;
>  	if (pfn > (pvmw->pfn >> PVMW_PFN_SHIFT) + pvmw->nr_pages - 1)
> @@ -255,6 +268,8 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>
>  				if (!softleaf_is_migration(entry) ||
>  				    !check_pmd(softleaf_to_pfn(entry),
> +					       softleaf_is_device_private(entry) ||
> +					       softleaf_is_migration_device_private(entry),
>  					       pvmw))
>  					return not_found(pvmw);
>  				return true;
> @@ -262,7 +277,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>  			if (likely(pmd_trans_huge(pmde))) {
>  				if (pvmw->flags & PVMW_MIGRATION)
>  					return not_found(pvmw);
> -				if (!check_pmd(pmd_pfn(pmde), pvmw))
> +				if (!check_pmd(pmd_pfn(pmde), false, pvmw))
>  					return not_found(pvmw);
>  				return true;
>  			}

It seems to me that you can add a new flag like “bool is_device_private” to
indicate whether pfn is a device private index instead of pfn without
manipulating pvmw->pfn itself.

Best Regards,
Yan, Zi