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

Jordan Niethe
2026-01-20T23:34:58Z

Hi,

On 21/1/26 10:06, Zi Yan wrote:
> On 20 Jan 2026, at 18:02, Jordan Niethe wrote:
>
>> Hi,
>>
>> On 21/1/26 09:53, Zi Yan wrote:
>>> On 20 Jan 2026, at 17:33, Jordan Niethe wrote:
>>>
>>>> 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.
>>>
>>> That is why I asked for a helper function like page_vma_walk_pfn(pvmw) to
>>> return the converted pfn instead of pvmw->pfn directly. You can add a comment
>>> to ask people to use helper function and even mark pvmw->pfn /* do not use
>>> directly */.
>>
>> Yeah I agree that is a good idea.
>>
>>>
>>> In addition, your patch manipulates pfn by left shifting it by 1. Are you sure
>>> there is no weird arch having pfns with bit 63 being 1? Your change could
>>> break it, right?
>>
>> Currently for migrate pfns we left shift by pfns by MIGRATE_PFN_SHIFT (6), so I
>> thought doing something similiar here should be safe.
>
> Yeah, but that limits to archs supporting HMM. page_vma_mapped_walk is used
> by almost every arch, so it has a broader impact.

That is a good point.

I see a few options:

- On every arch we can assume SWP_PFN_BITS? I could add a sanity check that we
have an extra bit on top of SWP_PFN_BITS within an unsigned long.
- We could define PVMW_PFN_SHIFT as 0 if !CONFIG_MIGRATION as the flag is not
required.
- Instead of modifying pvmw->pfn we could use pvmw->flags but that has the
issues of separating the offset type and offset.

Thanks,
Jordan.

>
> Best Regards,
> Yan, Zi