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

[Matthew Brost]

On Wed, Jul 23, 2025 at 05:34:17PM +0200, Francois Dugast wrote:
> 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.
> 

This is also a functional change. Once THP device pages are enabled (for
performance), we will encounter device-private swap entries in PMDs. At
that point, the correct behavior is to populate HMM PFNs from the swap
entry when dev_private_owner matches; otherwise, trigger a fault if the
HMM range-walk input requests one, or skip it in the non-faulting case.

It’s harmless to merge this patch before THP device pages are enabled,
since with the current code base we never find device-private swap
entries in PMDs.

I'd include something like the above explanation in the patch commit
message, or in code comments if needed.

Matt

> > 
> > > --- 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);
> >