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

[Francois Dugast]

On Tue, Jul 22, 2025 at 01:07:21PM -0700, Andrew Morton wrote:
> On Tue, 22 Jul 2025 21:34:45 +0200 Francois Dugast <francois.dugast@xxxxxxxxx> 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().
> 
> Please always tell us why a patch does something, not only what it does.

Sure, let me improve this in the next version.

> 
> > 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 infer from this paragraph that this patch is a performance
> optimization?  Have its effects been measured?

Yes, this performance optimization would come from avoiding the loop
over the range but it has neither been properly tested nor measured
yet.

> 
> > --- 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.
> > +		 */
> 
> Similarly, this tells us "what" it does, which is fairly obvious from
> the code itself.  What is not obvious from the code is the "why".

Indeed, will fix.

> 
> > +		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;
> > +			}
> > +
> > +			return 0;
> > +		}
> > +
> >  		if (hmm_range_need_fault(hmm_vma_walk, hmm_pfns, npages, 0))
> >  			return -EFAULT;
> >  		return hmm_pfns_fill(start, end, range, HMM_PFN_ERROR);
>