linux-kernel - Re: [v5 03/15] mm/rmap: extend rmap and migration support device-private entries

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Message-ID: <c829f1fb-911a-41c8-9f78-371781f8e081@nvidia.com>
Date: Mon, 15 Sep 2025 12:37:51 +1000
From: Balbir Singh <balbirs@...dia.com>
To: David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Cc: damon@...ts.linux.dev, dri-devel@...ts.freedesktop.org,
 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: [v5 03/15] mm/rmap: extend rmap and migration support
 device-private entries

On 9/11/25 22:04, David Hildenbrand wrote:
> On 08.09.25 02:04, Balbir Singh wrote:
> 
> subject:
> 
> "mm/rmap: rmap and migration support for device-private PMD entries"
> 
> 
>> Add device-private THP support to reverse mapping infrastructure,
>> enabling proper handling during migration and walk operations.
>>
>> The key changes are:
>> - add_migration_pmd()/remove_migration_pmd(): Handle device-private
>>    entries during folio migration and splitting
>> - page_vma_mapped_walk(): Recognize device-private THP entries during
>>    VMA traversal operations
>>
>> This change supports folio splitting and migration operations on
>> device-private entries.
>>
>> 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>
>> ---
> 
> [...]
> 
> 
>> +++ b/mm/page_vma_mapped.c
>> @@ -250,12 +250,11 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>>               pvmw->ptl = pmd_lock(mm, pvmw->pmd);
>>               pmde = *pvmw->pmd;
>>               if (!pmd_present(pmde)) {
>> -                swp_entry_t entry;
>> +                swp_entry_t entry = pmd_to_swp_entry(pmde);
>>                     if (!thp_migration_supported() ||
>>                       !(pvmw->flags & PVMW_MIGRATION))
>>                       return not_found(pvmw);
>> -                entry = pmd_to_swp_entry(pmde);
>>                   if (!is_migration_entry(entry) ||
>>                       !check_pmd(swp_offset_pfn(entry), pvmw))
>>                       return not_found(pvmw);
> 
> Why this change? Looks unrelated.
> 

I fixed up and removed the new flag I introduced in v3. Will fix/undo


>> @@ -277,6 +276,15 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>>                * cannot return prematurely, while zap_huge_pmd() has
>>                * cleared *pmd but not decremented compound_mapcount().
> 
> Reminder to self: cleanup compound_mapcount() leftovers
> 
>>                */
>> +            swp_entry_t entry;
>> +
>> +            entry = pmd_to_swp_entry(pmde);
> 
> swp_entry_t entry = pmd_to_swp_entry(pmde);
> 

Ack

>> +
>> +            if (is_device_private_entry(entry)) {
>> +                pvmw->ptl = pmd_lock(mm, pvmw->pmd);
>> +                return true;
>> +            }
>> +
>>               if ((pvmw->flags & PVMW_SYNC) &&
>>                   thp_vma_suitable_order(vma, pvmw->address,
>>                              PMD_ORDER) &&
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index 236ceff5b276..6de1baf7a4f1 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -1063,8 +1063,10 @@ static int page_vma_mkclean_one(struct page_vma_mapped_walk *pvmw)
>>           } else {
>>   #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>>               pmd_t *pmd = pvmw->pmd;
>> -            pmd_t entry;
>> +            pmd_t entry = pmdp_get(pmd);
>>   +            if (!pmd_present(entry))
>> +                continue;
>>               if (!pmd_dirty(*pmd) && !pmd_write(*pmd))
>>                   continue;
>>   
> 
> If you just did a pmdp_get() you should use it in these functions as well. If not (cleanup later), do a straight *pmd like the others.
> 
> 

Ack

> 
> Apart from that nothing jumped at me.
> 

Thanks for the review,
Balbir