linux-kernel - Re: [v4 05/15] mm/migrate_device: handle partially mapped folios during collection

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Message-ID: <6a178e78-9ccd-4845-b4ca-1e84f7d31b91@nvidia.com>
Date: Wed, 3 Sep 2025 16:05:03 +1000
From: Balbir Singh <balbirs@...dia.com>
To: Mika Penttilä <mpenttil@...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>,
 David Hildenbrand <david@...hat.com>, 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>,
 Matthew Brost <matthew.brost@...el.com>,
 Francois Dugast <francois.dugast@...el.com>
Subject: Re: [v4 05/15] mm/migrate_device: handle partially mapped folios
 during collection

On 9/3/25 14:40, Mika Penttilä wrote:
> Hi,
> 
> On 9/3/25 04:18, Balbir Singh wrote:
> 
>> Extend migrate_vma_collect_pmd() to handle partially mapped large
>> folios that require splitting before migration can proceed.
>>
>> During PTE walk in the collection phase, if a large folio is only
>> partially mapped in the migration range, it must be split to ensure
>> the folio is correctly migrated.
>>

<snip>

>> +
>> +				/*
>> +				 * The reason for finding pmd present with a
>> +				 * large folio for the pte is partial unmaps.
>> +				 * Split the folio now for the migration to be
>> +				 * handled correctly
>> +				 */
> 
> There are other reasons like vma splits for various reasons.
> 

Yes, David had pointed that out as well, I meant to cleanup the comment change
"The" to "One", I missed addressing it in the refactor, but easy to do

Thanks,
Balbir Singh