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

Alistair Popple
2026-01-23T02:03:18Z

On 2026-01-21 at 10:06 +1100, Zi Yan <ziy@nvidia.com> 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.

We need to be a bit careful by what we mean when we say "HMM" in the kernel.

Specifically MIGRATE_PFN_SHIFT is used with migrate_vma/migrate_device, which
is the migration half of "HMM" which does depend on CONFIG_DEVICE_MIGRATION or
really just CONFIG_ZONE_DEVICE making it somewhat arch specific.

However hmm_range_fault() does something similar - see the definition of
hmm_pfn_flags - it actually steals the top 11 bits of a pfn for flags, and it is
not architecture specific. It only depends on CONFIG_MMU.

Now I'm not saying this implies it actually works on all architectures as I
agree the page_vma_mapped_walk code is used much more widely. Rather I'm just
pointing out if there are issues with some architectures using high PFN bits
then we likely have a problem here too :-)

- Alistair

> Best Regards,
> Yan, Zi