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

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Message-ID: <aIG+gqY2YRqBeKlh@lstrano-desk.jf.intel.com>
Date: Wed, 23 Jul 2025 22:02:58 -0700
From: Matthew Brost <matthew.brost@...el.com>
To: Balbir Singh <balbirs@...dia.com>, <jgg@...dia.com>, <leonro@...dia.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>
Subject: Re: [PATCH] mm/hmm: Do not fault in device private pages owned by
 the caller

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.

Matt

> > 
> > 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