linux-kernel - Re: [PATCH] fixup: mm/rmap: extend rmap and migration support device-private entries

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Message-ID: <5b6bd015-538e-471e-b2bb-4a070bdc4c64@nvidia.com>
Date: Tue, 18 Nov 2025 09:39:16 +1100
From: Balbir Singh <balbirs@...dia.com>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 dri-devel@...ts.freedesktop.org
Cc: Andrew Morton <akpm@...ux-foundation.org>, Zi Yan <ziy@...dia.com>,
 Joshua Hahn <joshua.hahnjy@...il.com>, Rakie Kim <rakie.kim@...com>,
 Byungchul Park <byungchul@...com>, Gregory Price <gourry@...rry.net>,
 Ying Huang <ying.huang@...ux.alibaba.com>,
 Alistair Popple <apopple@...dia.com>, Oscar Salvador <osalvador@...e.de>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Nico Pache <npache@...hat.com>,
 Ryan Roberts <ryan.roberts@....com>, Dev Jain <dev.jain@....com>,
 Barry Song <baohua@...nel.org>, Lyude Paul <lyude@...hat.com>,
 Danilo Krummrich <dakr@...nel.org>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, Ralph Campbell <rcampbell@...dia.com>,
 Mika Penttilä <mpenttil@...hat.com>,
 Matthew Brost <matthew.brost@...el.com>,
 Francois Dugast <francois.dugast@...el.com>
Subject: Re: [PATCH] fixup: mm/rmap: extend rmap and migration support
 device-private entries

On 11/17/25 23:58, David Hildenbrand (Red Hat) wrote:
> On 15.11.25 01:28, Balbir Singh wrote:
>> Follow the pattern used in remove_migration_pte() in
>> remove_migration_pmd(). Process the migration entries and if the entry
>> type is device private, override the pmde with a device private entry
>> and set the soft dirty and uffd_wp bits with the pmd_swp_mksoft_dirty
>> and pmd_swp_mkuffd_wp
>>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Cc: David Hildenbrand <david@...hat.com>
>> Cc: Zi Yan <ziy@...dia.com>
>> Cc: Joshua Hahn <joshua.hahnjy@...il.com>
>> Cc: Rakie Kim <rakie.kim@...com>
>> Cc: Byungchul Park <byungchul@...com>
>> Cc: Gregory Price <gourry@...rry.net>
>> Cc: Ying Huang <ying.huang@...ux.alibaba.com>
>> Cc: Alistair Popple <apopple@...dia.com>
>> Cc: Oscar Salvador <osalvador@...e.de>
>> Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>> Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>
>> Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>
>> Cc: Nico Pache <npache@...hat.com>
>> Cc: Ryan Roberts <ryan.roberts@....com>
>> Cc: Dev Jain <dev.jain@....com>
>> Cc: Barry Song <baohua@...nel.org>
>> Cc: Lyude Paul <lyude@...hat.com>
>> Cc: Danilo Krummrich <dakr@...nel.org>
>> Cc: David Airlie <airlied@...il.com>
>> Cc: Simona Vetter <simona@...ll.ch>
>> Cc: Ralph Campbell <rcampbell@...dia.com>
>> Cc: Mika Penttilä <mpenttil@...hat.com>
>> Cc: Matthew Brost <matthew.brost@...el.com>
>> Cc: Francois Dugast <francois.dugast@...el.com>
>>
>> Signed-off-by: Balbir Singh <balbirs@...dia.com>
>> ---
>> This fixup should be squashed into the patch "mm/rmap: extend rmap and
>> migration support" of mm/mm-unstable
>>
>>   mm/huge_memory.c | 27 +++++++++++++++++----------
>>   1 file changed, 17 insertions(+), 10 deletions(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 9dda8c48daca..50ba458efcab 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -4698,16 +4698,6 @@ void remove_migration_pmd(struct page_vma_mapped_walk *pvmw, struct page *new)
>>       folio_get(folio);
>>       pmde = folio_mk_pmd(folio, READ_ONCE(vma->vm_page_prot));
>>   -    if (folio_is_device_private(folio)) {
>> -        if (pmd_write(pmde))
>> -            entry = make_writable_device_private_entry(
>> -                            page_to_pfn(new));
>> -        else
>> -            entry = make_readable_device_private_entry(
>> -                            page_to_pfn(new));
>> -        pmde = swp_entry_to_pmd(entry);
>> -    }
>> -
>>       if (pmd_swp_soft_dirty(*pvmw->pmd))
>>           pmde = pmd_mksoft_dirty(pmde);
>>       if (is_writable_migration_entry(entry))
>> @@ -4720,6 +4710,23 @@ void remove_migration_pmd(struct page_vma_mapped_walk *pvmw, struct page *new)
>>       if (folio_test_dirty(folio) && is_migration_entry_dirty(entry))
>>           pmde = pmd_mkdirty(pmde);
>>   +    if (folio_is_device_private(folio)) {
>> +        swp_entry_t entry;
> 
> It's a bit nasty to have the same variable shadowed here.
> 
> We could reuse the existing entry by handling the code more similar to remove_migration_pte(): determine RMAP_EXCLUSIVE earlier.
> 
>> +
>> +        if (pmd_write(pmde))
>> +            entry = make_writable_device_private_entry(
>> +                            page_to_pfn(new));
>> +        else
>> +            entry = make_readable_device_private_entry(
>> +                            page_to_pfn(new));
>> +        pmde = swp_entry_to_pmd(entry);
>> +
>> +        if (pmd_swp_soft_dirty(*pvmw->pmd))
>> +            pmde = pmd_swp_mksoft_dirty(pmde);
>> +        if (pmd_swp_uffd_wp(*pvmw->pmd))
>> +            pmde = pmd_swp_mkuffd_wp(pmde);
>> +    }
>> +
>>       if (folio_test_anon(folio)) {
>>           rmap_t rmap_flags = RMAP_NONE;
>>   
> 
> I guess at some point we could separate both parts completely (no need to do all this work on pmdb before the folio_is_device_private(folio) check, so this could be
> 
> if (folio_is_device_private(folio)) {
>     ...
> } else {
>     entry = pmd_to_swp_entry(*pvmw->pmd);
>     folio_get(folio);
>     ...
> }
> 
> That is something for another day though, and remove_migration_pte() should be cleaned up then as well.
> 

Agreed and Thanks for the review!

Balbir