Re: [PATCH v2 00/11] Remove device private pages from physical address space

[Matthew Brost <matthew.brost@intel.com>]

On Wed, Jan 07, 2026 at 08:18:12PM +1100, Jordan Niethe wrote:
> Today, 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 [0].
> 
> This series changes device private memory so that it does not require
> allocation of physical address space and these problems are avoided.
> Instead of using the physical address space, we introduce a "device
> private address space" and allocate from there.
> 
> A consequence of placing the device private pages outside of the
> physical address space is that they no longer have a PFN. However, it is
> still necessary to be able to look up a corresponding device private
> page from a device private PTE entry, which means that we still require
> some way to index into this device private address space. Instead of a
> PFN, device private pages use an offset into this device private address
> space to look up device private struct pages.
> 
> The problem that then needs to be addressed is how to avoid confusing
> these device private offsets with PFNs. It is the inherent limited usage
> of the device private pages themselves which make this possible. A
> device private page is only used for userspace mappings, we do not need
> to be concerned with them being used within the mm more broadly. This
> means that the only way that the core kernel looks up these pages is via
> the page table, where their PTE already indicates if they refer to a
> device private page via their swap type, e.g.  SWP_DEVICE_WRITE. We can
> use this information to determine if the PTE contains a PFN which should
> be looked up in the page map, or a device private offset which should be
> looked up elsewhere.
> 
> This applies when we are creating PTE entries for device private pages -
> because they have their own type there are already must be handled
> separately, so it is a small step to convert them to a device private
> PFN now too.
> 
> The first part of the series updates callers where device private
> offsets might now be encountered to track this extra state.
> 
> The last patch contains the bulk of the work where we change how we
> convert between device private pages to device private offsets and then
> use a new interface for allocating device private pages without the need
> for reserving physical address space.
> 
> By removing the device private pages from the physical address space,
> this series also opens up the possibility to moving away from tracking
> device private memory using struct pages in the future. This is
> desirable as on systems with large amounts of memory these device
> private struct pages use a signifiant amount of memory and take a
> significant amount of time to initialize.
> 
> *** Changes in v2 ***
> 
> The most significant change in v2 is addressing code paths that are
> common between MEMORY_DEVICE_PRIVATE and MEMORY_DEVICE_COHERENT devices.
> 
> This had been overlooked in previous revisions.
> 
> To do this we introduce a migrate_pfn_from_page() helper which will call
> device_private_offset_to_page() and set the MIGRATE_PFN_DEVICE_PRIVATE
> flag if required.
> 
> In places where we could have a device private offset
> (MEMORY_DEVICE_PRIVATE) or a pfn (MEMORY_DEVICE_COHERENT) we update to
> use an mpfn to disambiguate.  This includes some users in the drivers
> and migrate_device_{pfns,range}().
> 
> Seeking opinions on using the mpfns like this or if a new type would be
> preferred.
> 
>   - mm/migrate_device: Introduce migrate_pfn_from_page() helper
>     - New to series
> 
>   - drm/amdkfd: Use migrate pfns internally
>     - New to series
> 
>   - mm/migrate_device: Make migrate_device_{pfns,range}() take mpfns
>     - New to series
> 
>   - mm/migrate_device: Add migrate PFN flag to track device private pages
>     - Update for migrate_pfn_from_page()
>     - Rename to MIGRATE_PFN_DEVICE_PRIVATE
>     - drm/amd: Check adev->gmc.xgmi.connected_to_cpu
>     - lib/test_hmm.c: Check chunk->pagemap.type == MEMORY_DEVICE_PRIVATE
> 
>   - mm: Add helpers to create migration entries from struct pages
>     - Add a flags param
> 
>   - mm: Add a new swap type for migration entries of device private pages
>     - Add softleaf_is_migration_device_private_read()
> 
>   - mm: Add helpers to create device private entries from struct pages
>     - Add a flags param
> 
>   - mm: Remove device private pages from the physical address space
>     - Make sure last member of struct dev_pagemap remains DECLARE_FLEX_ARRAY(struct range, ranges);
> 
> Testing:
> - selftests/mm/hmm-tests on an amd64 VM
> 
> * NOTE: I will need help in testing the driver changes *
> 

Thanks for the series. For some reason Intel's CI couldn't apply this
series to drm-tip to get results [1]. I'll manually apply this and run all
our SVM tests and get back you on results + review the changes here. For
future reference if you want to use our CI system, the series must apply
to drm-tip, feel free to rebase this series and just send to intel-xe
list if you want CI results.

I was also wondering if Nvidia could help review one our core MM patches
[2] which is gating enabling 2M device pages too?

Matt

[1] https://patchwork.freedesktop.org/series/159738/
[2] https://patchwork.freedesktop.org/patch/694775/?series=159119&rev=1 

> Revisions:
> - RFC: https://lore.kernel.org/all/20251128044146.80050-1-jniethe@nvidia.com/
> - v1: https://lore.kernel.org/all/20251231043154.42931-1-jniethe@nvidia.com/
> 
> [0] https://lore.kernel.org/lkml/CAMj1kXFZ=4hLL1w6iCV5O5uVoVLHAJbc0rr40j24ObenAjXe9w@mail.gmail.com/
> 
> Jordan Niethe (11):
>   mm/migrate_device: Introduce migrate_pfn_from_page() helper
>   drm/amdkfd: Use migrate pfns internally
>   mm/migrate_device: Make migrate_device_{pfns,range}() take mpfns
>   mm/migrate_device: Add migrate PFN flag to track device private pages
>   mm/page_vma_mapped: Add flags to page_vma_mapped_walk::pfn to track
>     device private pages
>   mm: Add helpers to create migration entries from struct pages
>   mm: Add a new swap type for migration entries of device private pages
>   mm: Add helpers to create device private entries from struct pages
>   mm/util: Add flag to track device private pages in page snapshots
>   mm/hmm: Add flag to track device private pages
>   mm: Remove device private pages from the physical address space
> 
>  Documentation/mm/hmm.rst                 |  11 +-
>  arch/powerpc/kvm/book3s_hv_uvmem.c       |  43 ++---
>  drivers/gpu/drm/amd/amdkfd/kfd_migrate.c |  45 +++---
>  drivers/gpu/drm/amd/amdkfd/kfd_migrate.h |   2 +-
>  drivers/gpu/drm/drm_pagemap.c            |  11 +-
>  drivers/gpu/drm/nouveau/nouveau_dmem.c   |  45 ++----
>  drivers/gpu/drm/xe/xe_svm.c              |  37 ++---
>  fs/proc/page.c                           |   6 +-
>  include/drm/drm_pagemap.h                |   8 +-
>  include/linux/hmm.h                      |   7 +-
>  include/linux/leafops.h                  | 116 ++++++++++++--
>  include/linux/memremap.h                 |  64 +++++++-
>  include/linux/migrate.h                  |  23 ++-
>  include/linux/mm.h                       |   9 +-
>  include/linux/rmap.h                     |  33 +++-
>  include/linux/swap.h                     |   8 +-
>  include/linux/swapops.h                  | 136 ++++++++++++++++
>  lib/test_hmm.c                           |  86 ++++++----
>  mm/debug.c                               |   9 +-
>  mm/hmm.c                                 |   5 +-
>  mm/huge_memory.c                         |  43 ++---
>  mm/hugetlb.c                             |  15 +-
>  mm/memory.c                              |   5 +-
>  mm/memremap.c                            | 193 ++++++++++++++++++-----
>  mm/migrate.c                             |   6 +-
>  mm/migrate_device.c                      |  76 +++++----
>  mm/mm_init.c                             |   8 +-
>  mm/mprotect.c                            |  10 +-
>  mm/page_vma_mapped.c                     |  32 +++-
>  mm/rmap.c                                |  59 ++++---
>  mm/util.c                                |   8 +-
>  mm/vmscan.c                              |   2 +-
>  32 files changed, 822 insertions(+), 339 deletions(-)
> 
> 
> base-commit: f8f9c1f4d0c7a64600e2ca312dec824a0bc2f1da
> -- 
> 2.34.1
>