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Date: Thu, 11 Apr 2024 09:32:04 -0300
From: Jason Gunthorpe <jgg@nvidia.com>
To: Alistair Popple <apopple@nvidia.com>
Cc: linux-mm@kvack.org, david@fromorbit.com, dan.j.williams@intel.com,
	jhubbard@nvidia.com, rcampbell@nvidia.com, willy@infradead.org,
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Subject: Re: [RFC 07/10] mm: Allow compound zone device pages
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On Thu, Apr 11, 2024 at 10:57:28AM +1000, Alistair Popple wrote:
> Zone device pages are used to represent various type of device memory
> managed by device drivers. Currently compound zone device pages are
> not supported. This is because MEMORY_DEVICE_FS_DAX pages are the only
> user of higher order zone device pages and have their own page
> reference counting.
> 
> A future change will unify FS DAX reference counting with normal page
> reference counting rules and remove the special FS DAX reference
> counting. Supporting that requires compound zone device pages.
> 
> Supporting compound zone device pages requires compound_head() to
> distinguish between head and tail pages whilst still preserving the
> special struct page fields that are specific to zone device pages.
> 
> A tail page is distinguished by having bit zero being set in
> page->compound_head, with the remaining bits pointing to the head
> page. For zone device pages page->compound_head is shared with
> page->pgmap.
> 
> The page->pgmap field is common to all pages within a memory section.
> Therefore pgmap is the same for both head and tail pages and we can
> use the same scheme to distinguish tail pages. To obtain the pgmap for
> a tail page a new accessor is introduced to fetch it from
> compound_head.
> 
> Signed-off-by: Alistair Popple <apopple@nvidia.com>
> ---
>  drivers/gpu/drm/nouveau/nouveau_dmem.c |  2 +-
>  drivers/pci/p2pdma.c                   |  2 +-
>  include/linux/memremap.h               | 12 +++++++++---
>  include/linux/migrate.h                |  2 +-
>  lib/test_hmm.c                         |  2 +-
>  mm/hmm.c                               |  2 +-
>  mm/memory.c                            |  2 +-
>  mm/memremap.c                          |  6 +++---
>  mm/migrate_device.c                    |  4 ++--
>  9 files changed, 20 insertions(+), 14 deletions(-)

Makes sense to me

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>

Jason