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

Alistair Popple
2025-11-30T23:23:43Z

On 2025-11-29 at 06:22 +1100, Matthew Brost <matthew.brost@intel.com> wrote...
> 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.

It most certainly will :-) I think Jordan called that out above but we wanted
to get the design right before spending too much time updating drivers. That
said I don't think the driver changes should be extensive, but let us know if
you disagree.

> - 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.

Thanks!

> - 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.

Our aim here is to get some review of the design and the patches/implementation
for the 6.19 cycle but I agree that this will need to get rebased on top of
Balbir's series.

- Alistair

> 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
> >