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

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Message-ID: <8ffcbd7a-511e-499b-919d-0c56ccc3c63b@lucifer.local>
Date: Thu, 13 Nov 2025 16:02:15 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Francois Dugast <francois.dugast@...el.com>
Cc: Balbir Singh <balbirs@...dia.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, 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>,
        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 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)?

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.

Cheers, Lorenzo