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

Jordan Niethe
2026-01-20T22:33:46Z

On 14/1/26 07:04, Zi Yan wrote:
> 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.

ack.

>
>> +
>> 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.

We could do it like that, however my concern with using a new param was that
storing this info seperately might make it easier to misuse a device private
index as a regular pfn.

It seemed like it could be easy to overlook both when creating the pvmw and
then when accessing the pfn.

Thanks,
Jordan.

>
> Best Regards,
> Yan, Zi