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

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Message-ID: <ffac73b3-3c2f-402a-beb3-a98ba92c5335@redhat.com>
Date: Wed, 3 Sep 2025 11:26:08 +0300
From: Mika Penttilä <mpenttil@...hat.com>
To: Balbir Singh <balbirs@...dia.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 09:05, Balbir Singh wrote:

> 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

And of course now you split all mTHPs as well, which is different what we do today
(ignoring). Splitting might be the right thing to do, but maybe worth mentioning.

> Thanks,
> Balbir Singh
>
--Mika