Message-ID: <anu425gyhd5w3boribueuyckjlg73r356kkoy6iv6dcsy6b5ar@5qnbvtymgmje>
Date: Mon, 1 Dec 2025 10:23:32 +1100
From: Alistair Popple <apopple@...dia.com>
To: Matthew Brost <matthew.brost@...el.com>
Cc: Jordan Niethe <jniethe@...dia.com>, linux-mm@...ck.org, 
	balbirs@...dia.com, akpm@...ux-foundation.org, linux-kernel@...r.kernel.org, 
	dri-devel@...ts.freedesktop.org, david@...hat.com, ziy@...dia.com, lorenzo.stoakes@...cle.com, 
	lyude@...hat.com, dakr@...nel.org, airlied@...il.com, simona@...ll.ch, 
	rcampbell@...dia.com, mpenttil@...hat.com, jgg@...dia.com, willy@...radead.org
Subject: Re: [RFC PATCH 0/6] Remove device private pages from physical
 address space

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

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