linux-kernel - Re: [v7 11/16] mm/migrate_device: add THP splitting during migration

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Message-ID: <76750d20-cdfe-41bb-a228-9b3f171675ec@kernel.org>
Date: Tue, 10 Feb 2026 10:39:48 +0100
From: "David Hildenbrand (Arm)" <david@...nel.org>
To: Balbir Singh <balbirs@...dia.com>, linux-kernel@...r.kernel.org,
 dri-devel@...ts.freedesktop.org, linux-mm@...ck.org
Cc: 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: [v7 11/16] mm/migrate_device: add THP splitting during migration

On 2/9/26 22:57, Balbir Singh wrote:
> On 2/10/26 03:00, David Hildenbrand (Arm) wrote:
>>
>>> index 8c95a658b3ec..022b0729f826 100644
>>> --- a/mm/huge_memory.c
>>> +++ b/mm/huge_memory.c
>>> @@ -3463,15 +3463,6 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
>>>            new_folio->mapping = folio->mapping;
>>>            new_folio->index = folio->index + i;
>>>    -        /*
>>> -         * page->private should not be set in tail pages. Fix up and warn once
>>> -         * if private is unexpectedly set.
>>> -         */
>>> -        if (unlikely(new_folio->private)) {
>>> -            VM_WARN_ON_ONCE_PAGE(true, new_head);
>>> -            new_folio->private = NULL;
>>> -        }
>>> -
>>
>> Balbir, why did you drop this check?
>>

Thanks for your reply.

> 
> Are we running into this somewhere? This change clearly seems unrelated to the intent of the
> patch that made this change (by me). 

We stumbled over its surprise removal while discussing something related.

I thought you removed it because of some ZONE_DEVICE magic; the check 
has been proven helpful to catch bugs in the past.

We always document carefully what our patches do, and why; and avoid 
doing unrelated things in our patches where possible.

> I was seeing new_folio->private as NULL everywhere during
> my testing and so I removed the check, happy to bring this defensive test back.

I'll take care of re-adding it if is still useful after the other 
->private changes we are planning.

-- 
Cheers,

David