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

Jordan Niethe
2025-12-02T01:20:25Z

Hi,

On 29/11/25 05:36, Matthew Brost wrote:
> 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.

Thanks Matt, I agree. There will be a few more updates to this
series for 2MB device pages - I'll send the next revision on top of that
support.

Jordan.

>
> 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
>>