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

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

On Mon, Dec 01, 2025 at 10:23:32AM +1100, Alistair Popple wrote:
> 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

I don't always read.

> 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 did a quick look, and I believe it pretty minor (e.g., pfn_to_page is used a
few places for device pages which would need a refactor, etc...). Maybe
a bit more, we will find out but not too concerned.

> 
> > - 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.

+1. Will be on the lookout for the next post and pull into 6.19 DRM tree
and at least test out the Intel stuffi + send fixes if needed.

I can enable both of you for Intel CI too, just include intel-xe list on
next post and it will get kicked off and you can find the results on
patchworks.

Matt

> 
>  - 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
> > >