Re: [RFC PATCH 0/6] Remove device private pages from physical address space

Matthew Brost
2025-11-28T19:23:03Z

On Fri, Nov 28, 2025 at 03:41:40PM +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 RFC 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. This leads to
> the idea of a device private PFN. This is like a PFN but instead of
> associating memory in the physical address space with a struct page, it
> associates device memory in the device private address space with a
> device private struct page.
>
> The problem that then needs to be addressed is how to avoid confusing
> these device private PFNs with the regular 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 normal PFN which should be looked up in the page map, or a
> device private PFN 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 PFNs
> 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 PFNs and then use
> a new interface for allocating device private pages without the need for
> reserving physical address space.
>
> For the purposes of the RFC changes have been limited to test_hmm.c
> updates to the other drivers will be included in the next revision.
>
> This would include updating existing users of memremap_pages() to use
> memremap_device_private_pagemap() instead to allocate device private
> pages. This also means they would no longer need to call
> request_free_mem_region(). An equivalent of devm_memremap_pages() will
> also be necessary.
>
> Users of the migrate_vma() interface will also need to be updated to be
> aware these device private PFNs.
>
> By removing the device private pages from the physical address space,
> this RFC 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.

A couple things.

- I’m fairly certain that, briefly looking at this, it will break all
upstream DRM drivers (AMDKFD, Nouveau, Xe / GPUSVM) that use device
private pages. I looked into what I think conflicts with Xe / GPUSVM,
and I believe the impact is fairly minor. I’m happy to help by pulling
this code and fixing up our side.

- I’m fully on board with eventually moving to something that uses less
memory than struct page, and I’m happy to coordinate on future changes.

- Before we start coordinating on this patch set, should we hold off until
the 6.19 cycle, which includes 2M device pages from Balbir [1] (i.e.,
rebase this series on top of 6.19 once it includes 2M pages)? I suspect
that, given the scope of this series and Balbir’s, there will be some
conflicts.

Matt

[1] https://patchwork.freedesktop.org/series/152798/

>
> Testing:
> - selftests/mm/hmm-tests on an amd64 VM
>
> [0] https://lore.kernel.org/lkml/CAMj1kXFZ=4hLL1w6iCV5O5uVoVLHAJbc0rr40j24ObenAjXe9w@mail.gmail.com/
>
> Jordan Niethe (6):
> mm/hmm: Add flag to track device private PFNs
> mm/migrate_device: Add migrate PFN flag to track device private PFNs
> mm/page_vma_mapped: Add flags to page_vma_mapped_walk::pfn to track
> device private PFNs
> mm: Add a new swap type for migration entries with device private PFNs
> mm/util: Add flag to track device private PFNs in page snapshots
> mm: Remove device private pages from the physical address space
>
> Documentation/mm/hmm.rst | 9 +-
> fs/proc/page.c | 6 +-
> include/linux/hmm.h | 5 ++
> include/linux/memremap.h | 25 +++++-
> include/linux/migrate.h | 5 ++
> include/linux/mm.h | 9 +-
> include/linux/rmap.h | 33 +++++++-
> include/linux/swap.h | 8 +-
> include/linux/swapops.h | 102 +++++++++++++++++++++--
> lib/test_hmm.c | 66 ++++++++-------
> mm/debug.c | 9 +-
> mm/hmm.c | 2 +-
> mm/memory.c | 9 +-
> mm/memremap.c | 174 +++++++++++++++++++++++++++++----------
> mm/migrate.c | 6 +-
> mm/migrate_device.c | 44 ++++++----
> mm/mm_init.c | 8 +-
> mm/mprotect.c | 21 +++--
> mm/page_vma_mapped.c | 18 +++-
> mm/pagewalk.c | 2 +-
> mm/rmap.c | 68 ++++++++++-----
> mm/util.c | 8 +-
> mm/vmscan.c | 2 +-
> 23 files changed, 485 insertions(+), 154 deletions(-)
>
>
> base-commit: e1afacb68573c3cd0a3785c6b0508876cd3423bc
> --
> 2.34.1
>