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

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Message-ID: <8d02347d-6ca2-4dfc-ab98-940381fa8b86@nvidia.com>
Date: Wed, 3 Dec 2025 15:25:02 +1100
From: Balbir Singh <balbirs@...dia.com>
To: Jordan Niethe <jniethe@...dia.com>,
 Matthew Brost <matthew.brost@...el.com>
Cc: linux-mm@...ck.org, 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

On 12/2/25 12:20, Jordan Niethe wrote:
> 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),

Doesn't this break HMM_PFN_ORDER_SHIFT? The assumption is that we have 5 bits for
order

>>>       /*
>>>        * 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.
> 

I think it makes sense to build on top of v6.19 with THP support

Balbir