Re: [RFC PATCH 1/6] mm/hmm: Add flag to track device private PFNs

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

On Fri, Nov 28, 2025 at 03:41:41PM +1100, Jordan Niethe wrote:
> A future change will remove device private pages from the physical
> address space. This will mean that device private pages no longer have
> normal PFN and must be handled separately.
> 
> Prepare for this by adding a HMM_PFN_DEVICE_PRIVATE flag to indicate
> that a hmm_pfn contains a PFN for a device private page.
> 
> Signed-off-by: Jordan Niethe <jniethe@nvidia.com>
> Signed-off-by: Alistair Popple <apopple@nvidia.com>
> ---
>  include/linux/hmm.h | 2 ++
>  mm/hmm.c            | 2 +-
>  2 files changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/hmm.h b/include/linux/hmm.h
> index db75ffc949a7..df571fa75a44 100644
> --- a/include/linux/hmm.h
> +++ b/include/linux/hmm.h
> @@ -23,6 +23,7 @@ struct mmu_interval_notifier;
>   * HMM_PFN_WRITE - if the page memory can be written to (requires HMM_PFN_VALID)
>   * HMM_PFN_ERROR - accessing the pfn is impossible and the device should
>   *                 fail. ie poisoned memory, special pages, no vma, etc
> + * HMM_PFN_DEVICE_PRIVATE - the pfn field contains a DEVICE_PRIVATE pfn.
>   * HMM_PFN_P2PDMA - P2P page
>   * HMM_PFN_P2PDMA_BUS - Bus mapped P2P transfer
>   * HMM_PFN_DMA_MAPPED - Flag preserved on input-to-output transformation
> @@ -40,6 +41,7 @@ enum hmm_pfn_flags {
>  	HMM_PFN_VALID = 1UL << (BITS_PER_LONG - 1),
>  	HMM_PFN_WRITE = 1UL << (BITS_PER_LONG - 2),
>  	HMM_PFN_ERROR = 1UL << (BITS_PER_LONG - 3),
> +	HMM_PFN_DEVICE_PRIVATE = 1UL << (BITS_PER_LONG - 7),
>  	/*
>  	 * Sticky flags, carried from input to output,
>  	 * don't forget to update HMM_PFN_INOUT_FLAGS
> diff --git a/mm/hmm.c b/mm/hmm.c
> index 87562914670a..1cff68ade1d4 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -262,7 +262,7 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
>  		if (is_device_private_entry(entry) &&
>  		    page_pgmap(pfn_swap_entry_to_page(entry))->owner ==
>  		    range->dev_private_owner) {
> -			cpu_flags = HMM_PFN_VALID;
> +			cpu_flags = HMM_PFN_VALID | HMM_PFN_DEVICE_PRIVATE;

I think you’ll need to set this flag in hmm_vma_handle_absent_pmd as
well. That function handles 2M device pages. Support for 2M device
pages, I believe, will be included in the 6.19 PR, but
hmm_vma_handle_absent_pmd is already upstream.

Matt

>  			if (is_writable_device_private_entry(entry))
>  				cpu_flags |= HMM_PFN_WRITE;
>  			new_pfn_flags = swp_offset_pfn(entry) | cpu_flags;
> -- 
> 2.34.1
>