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

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

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.

Matt

> 
> > Matt
> 
> 
> --Mika
> 
> 
> >
> >>> Signed-off-by: Francois Dugast <francois.dugast@intel.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
>