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

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Message-ID: <75de1c7d-58d2-4a0f-b86c-b3dae65fb52f@nvidia.com>
Date: Tue, 2 Dec 2025 12:20:12 +1100
From: Jordan Niethe <jniethe@...dia.com>
To: Matthew Brost <matthew.brost@...el.com>
Cc: 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, apopple@...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 1/6] mm/hmm: Add flag to track device private PFNs

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@...dia.com>
>> Signed-off-by: Alistair Popple <apopple@...dia.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
>>