Re: [PATCH v5 13/13] mm: Remove device private pages from the physical address space

Jordan Niethe
2026-01-30T21:33:07Z

Hi,

On 30/1/26 22:10, Jordan Niethe wrote:
> The existing design of device private memory imposes limitations which
> render it non functional for certain systems and configurations where
> the physical address space is limited.
>
> Device private memory is implemented by first reserving a region of the
> physical address space. This is a problem. The physical address space is
> not a resource that is directly under the kernel's control. Availability
> of suitable physical address space is constrained by the underlying
> hardware and firmware and may not always be available.
>
> Device private memory assumes that it will be able to reserve a device
> memory sized chunk of physical address space. However, there is nothing
> guaranteeing that this will succeed, and there a number of factors that
> increase the likelihood of failure. We need to consider what else may
> exist in the physical address space. It is observed that certain VM
> configurations place very large PCI windows immediately after RAM. Large
> enough that there is no physical address space available at all for
> device private memory. This is more likely to occur on 43 bit physical
> width systems which have less physical address space.
>
> 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.
>
> Acked-by: Felix Kuehling <felix.kuehling@amd.com>
> Reviewed-by: Zi Yan <ziy@nvidia.com> # for MM changes
> Signed-off-by: Jordan Niethe <jniethe@nvidia.com>
> Signed-off-by: Alistair Popple <apopple@nvidia.com>
>
> ---

Hi Matthew - the latest intel-xe CI run is passing: https://patchwork.freedesktop.org/series/159738/#rev8

Would you mind reviewing the gpu/drm/xe changes here and in:
- [PATCH v5 01/13] mm/migrate_device: Introduce migrate_pfn_from_page() helper
- [PATCH v5 03/13] mm/migrate_device: Make migrate_device_{pfns,range}() take mpfns
- [PATCH v5 04/13] mm/migrate_device: Add migrate PFN flag to track device private pages

Thanks very much,
Jordan.