linux-kernel - Re: [PATCH] mm/huge_memory.c: introduce split_unmapped_folio_to

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Message-ID: <75d2254e-51ea-4e59-b706-86664e6c4bab@kernel.org>
Date: Thu, 13 Nov 2025 20:07:24 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Zi Yan <ziy@...dia.com>, Francois Dugast <francois.dugast@...el.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Balbir Singh <balbirs@...dia.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
 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>,
 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>
Subject: Re: [PATCH] mm/huge_memory.c: introduce split_unmapped_folio_to_order

On 13.11.25 17:24, Zi Yan wrote:
> On 13 Nov 2025, at 11:02, Lorenzo Stoakes wrote:
> 
>> On Thu, Nov 13, 2025 at 04:36:01PM +0100, Francois Dugast wrote:
>>> Hi Balbir,
>>>
>>> On Wed, Nov 12, 2025 at 03:46:33PM +1100, Balbir Singh wrote:
>>>> Unmapped was added as a parameter to __folio_split() and related
>>>> call sites to support splitting of folios already in the midst
>>>> of a migration. This special case arose for device private folio
>>>> migration since during migration there could be a disconnect between
>>>> source and destination on the folio size.
>>>>
>>>> Introduce split_unmapped_folio_to_order() to handle this special case.
>>>> This in turn removes the special casing introduced by the unmapped
>>>> parameter in __folio_split().
>>>
>>> Such a helper would be needed in drm_pagemap_migrate_to_devmem when
>>> reallocating a device folio to smaller pages.
>>>
>>> Could we export it (EXPORT_SYMBOL)?
> 
> drm_pagemap_migrate_to_devmem() is a function defined in tree, you
> just need to include huge_mm.h to use split_unmapped_folio_to_order().
> Why do you need to export split_unmapped_folio_to_order()?

I guess because DRM_GPUSVM is tristate, so can be built as a module. 
IIUC, that's where drm_pagemap_migrate_to_devmem() ends up.

> 
>>
>> As a rule we don't export things from core mm. And certainly not to non-GPL
>> modules.
>>
>> Unless David feels very differently or there's some enormously compelling
>> reason for it I'd really rather we didn't.

We'd need a pretty good reason to go down that path indeed :)

-- 
Cheers

David