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

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Message-ID: <9ae3e014-c7d0-4d58-af0e-925bcd9e4cfd@nvidia.com>
Date: Tue, 22 Jul 2025 09:34:13 +1000
From: Balbir Singh <balbirs@...dia.com>
To: Francois Dugast <francois.dugast@...el.com>
Cc: Matthew Brost <matthew.brost@...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 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]?

> +                       }
> +
> +                       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