linux-kernel - Re: [PATCH] mm/hmm: Do not fault in device private pages owned by the caller

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Message-ID: <368fa1c1-fccc-445d-bd22-0053fd2db29c@redhat.com>
Date: Thu, 24 Jul 2025 09:04:36 +0300
From: Mika Penttilä <mpenttil@...hat.com>
To: Matthew Brost <matthew.brost@...el.com>
Cc: Francois Dugast <francois.dugast@...el.com>, airlied@...il.com,
 akpm@...ux-foundation.org, apopple@...dia.com, baohua@...nel.org,
 baolin.wang@...ux.alibaba.com, dakr@...nel.org, david@...hat.com,
 donettom@...ux.ibm.com, jane.chu@...cle.com, jglisse@...hat.com,
 kherbst@...hat.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 lyude@...hat.com, peterx@...hat.com, ryan.roberts@....com, shuah@...nel.org,
 simona@...ll.ch, wangkefeng.wang@...wei.com, willy@...radead.org,
 ziy@...dia.com, Balbir Singh <balbirs@...dia.com>, jgg@...dia.com,
 Leon Romanovsky <leonro@...dia.com>
Subject: Re: [PATCH] mm/hmm: Do not fault in device private pages owned by the
 caller


On 7/24/25 08:57, Matthew Brost wrote:
> On Thu, Jul 24, 2025 at 08:46:11AM +0300, Mika Penttilä wrote:
>> On 7/24/25 08:02, Matthew Brost wrote:
>>> On Thu, Jul 24, 2025 at 10:25:11AM +1000, Balbir Singh wrote:
>>>> On 7/23/25 05:34, Francois Dugast wrote:
>>>>> When the PMD swap entry is device private and owned by the caller,
>>>>> skip the range faulting and instead just set the correct HMM PFNs.
>>>>> This is similar to the logic for PTEs in hmm_vma_handle_pte().
>>>>>
>>>>> For now, each hmm_pfns[i] entry is populated as it is currently done
>>>>> in hmm_vma_handle_pmd() but this might not be necessary. A follow-up
>>>>> optimization could be to make use of the order and skip populating
>>>>> subsequent PFNs.
>>>> I think we should test and remove these now
>>>>
>>> +Jason, Leon – perhaps either of you can provide insight into why
>>> hmm_vma_handle_pmd fully populates the HMM PFNs when a higher-order page
>>> is found.
>>>
>>> If we can be assured that changing this won’t break other parts of the
>>> kernel, I agree it should be removed. A snippet of documentation should
>>> also be added indicating that when higher-order PFNs are found,
>>> subsequent PFNs within the range will remain unpopulated. I can verify
>>> that GPU SVM works just fine without these PFNs being populated.
>> afaics the device can consume the range as smaller pages also, and some
>> hmm users depend on that.
>>
> Sure, but I think that should be fixed in the device code. If a
> large-order PFN is found, the subsequent PFNs can clearly be inferred.
> It's a micro-optimization here, but devices or callers capable of
> handling this properly shouldn't force a hacky, less optimal behavior on
> core code. If anything relies on the current behavior, we should fix it
> and ensure correctness.

Yes sure device code can be changed but meant to say we can't just
delete those lines without breaking existing users.


>
> Matt


--Mika


>
>>> Matt
>>
>> --Mika
>>
>>
>>>>> Signed-off-by: Francois Dugast <francois.dugast@...el.com>
>>>>> ---
>>>>>  mm/hmm.c | 25 +++++++++++++++++++++++++
>>>>>  1 file changed, 25 insertions(+)
>>>>>
>>>>> diff --git a/mm/hmm.c b/mm/hmm.c
>>>>> index f2415b4b2cdd..63ec1b18a656 100644
>>>>> --- a/mm/hmm.c
>>>>> +++ b/mm/hmm.c
>>>>> @@ -355,6 +355,31 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
>>>>>  	}
>>>>>  
>>>>>  	if (!pmd_present(pmd)) {
>>>>> +		swp_entry_t entry = pmd_to_swp_entry(pmd);
>>>>> +
>>>>> +		/*
>>>>> +		 * Don't fault in device private pages owned by the caller,
>>>>> +		 * just report the PFNs.
>>>>> +		 */
>>>>> +		if (is_device_private_entry(entry) &&
>>>>> +		    pfn_swap_entry_folio(entry)->pgmap->owner ==
>>>>> +		    range->dev_private_owner) {
>>>>> +			unsigned long cpu_flags = HMM_PFN_VALID |
>>>>> +				hmm_pfn_flags_order(PMD_SHIFT - PAGE_SHIFT);
>>>>> +			unsigned long pfn = swp_offset_pfn(entry);
>>>>> +			unsigned long i;
>>>>> +
>>>>> +			if (is_writable_device_private_entry(entry))
>>>>> +				cpu_flags |= HMM_PFN_WRITE;
>>>>> +
>>>>> +			for (i = 0; addr < end; addr += PAGE_SIZE, i++, pfn++) {
>>>>> +				hmm_pfns[i] &= HMM_PFN_INOUT_FLAGS;
>>>>> +				hmm_pfns[i] |= pfn | cpu_flags;
>>>>> +			}
>>>>> +
>>>> As discussed, can we remove these.
>>>>
>>>>> +			return 0;
>>>>> +		}
>>>> All of this be under CONFIG_ARCH_ENABLE_THP_MIGRATION
>>>>
>>>>> +
>>>>>  		if (hmm_range_need_fault(hmm_vma_walk, hmm_pfns, npages, 0))
>>>>>  			return -EFAULT;
>>>>>  		return hmm_pfns_fill(start, end, range, HMM_PFN_ERROR);
>>>>
>>>> Balbir Singh