linux-kernel - Re: [v1 resend 00/12] THP support for zone device page migration

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Message-ID: <aH7U0LVBf0NqCrWK@lstrano-desk.jf.intel.com>
Date: Mon, 21 Jul 2025 17:01:20 -0700
From: Matthew Brost <matthew.brost@...el.com>
To: Balbir Singh <balbirs@...dia.com>
CC: Francois Dugast <francois.dugast@...el.com>, <linux-mm@...ck.org>,
	<akpm@...ux-foundation.org>, <linux-kernel@...r.kernel.org>, Karol Herbst
	<kherbst@...hat.com>, Lyude Paul <lyude@...hat.com>, Danilo Krummrich
	<dakr@...nel.org>, David Airlie <airlied@...il.com>, Simona Vetter
	<simona@...ll.ch>, Jérôme Glisse <jglisse@...hat.com>,
	Shuah Khan <shuah@...nel.org>, David Hildenbrand <david@...hat.com>, "Barry
 Song" <baohua@...nel.org>, Baolin Wang <baolin.wang@...ux.alibaba.com>, "Ryan
 Roberts" <ryan.roberts@....com>, Matthew Wilcox <willy@...radead.org>, "Peter
 Xu" <peterx@...hat.com>, Zi Yan <ziy@...dia.com>, Kefeng Wang
	<wangkefeng.wang@...wei.com>, Jane Chu <jane.chu@...cle.com>, Alistair Popple
	<apopple@...dia.com>, Donet Tom <donettom@...ux.ibm.com>
Subject: Re: [v1 resend 00/12] THP support for zone device page migration

On Tue, Jul 22, 2025 at 09:34:13AM +1000, Balbir Singh wrote:
> On 7/21/25 21:42, Francois Dugast wrote:
> > On Fri, Jul 18, 2025 at 01:57:13PM +1000, Balbir Singh wrote:
> >> On 7/18/25 09:40, Matthew Brost wrote:
> >>> On Fri, Jul 04, 2025 at 09:34:59AM +1000, Balbir Singh wrote:
> >> ...
> >>>>
> >>>> The nouveau dmem code has been enhanced to use the new THP migration
> >>>> capability.
> >>>>
> >>>> Feedback from the RFC [2]:
> >>>>
> >>>
> >>> Thanks for the patches, results look very promising. I wanted to give
> >>> some quick feedback:
> >>>
> >>
> >> Are you seeing improvements with the patchset?
> >>
> >>> - You appear to have missed updating hmm_range_fault, specifically
> >>> hmm_vma_handle_pmd, to check for device-private entries and populate the
> >>> HMM PFNs accordingly. My colleague François has a fix for this if you're
> >>> interested.
> >>>
> >>
> >> Sure, please feel free to post them. 
> > 
> > Hi Balbir,
> > 
> > It seems we are missing this special handling in in hmm_vma_walk_pmd():
> > 
> > diff --git a/mm/hmm.c b/mm/hmm.c
> > index f2415b4b2cdd..449025f72b2f 100644
> > --- a/mm/hmm.c
> > +++ b/mm/hmm.c
> > @@ -355,6 +355,27 @@ 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 = pmd_to_hmm_pfn_flags(range, pmd);
> > +                       unsigned long pfn = swp_offset_pfn(entry);
> > +                       unsigned long i;
> > +
> > +                       for (i = 0; addr < end; addr += PAGE_SIZE, i++, pfn++) {
> > +                               hmm_pfns[i] &= HMM_PFN_INOUT_FLAGS;
> > +                               hmm_pfns[i] |= pfn | cpu_flags;
> 
> Won't we use hmm_pfn_to_map_order(), do we still need to populate each entry in the hmm_pfns[i]?
> 

I had the same question. hmm_vma_handle_pmd populates subsequent PFNs as
well, but this seems unnecessary. I removed both of these cases and my
code still worked. However, I haven't audited all kernel callers to
ensure this doesn't cause issues elsewhere.

Matt

> > +                       }
> > +
> > +                       return 0;
> > +               }
> > +
> 
> Thanks for the patch! If you could send it with a full sign-off, I can add it to my series while
> posting
> 
> Balbir Singh
> 
>