linux-kernel - Re: [PATCH v3 1/7] mm: Add folio_split

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Message-ID: <59fb1669-2908-4cab-b3c0-b97479da3fb9@kernel.org>
Date: Fri, 9 Jan 2026 14:19:16 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Francois Dugast <francois.dugast@...el.com>,
 intel-xe@...ts.freedesktop.org
Cc: dri-devel@...ts.freedesktop.org, Matthew Brost <matthew.brost@...el.com>,
 Balbir Singh <balbirs@...dia.com>, Andrew Morton
 <akpm@...ux-foundation.org>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Zi Yan <ziy@...dia.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>, Lance Yang <lance.yang@...ux.dev>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Alistair Popple <apopple@...dia.com>
Subject: Re: [PATCH v3 1/7] mm: Add folio_split_unref helper

On 1/9/26 09:54, Francois Dugast wrote:
> From: Matthew Brost <matthew.brost@...el.com>
> 
> Add folio_split_unref helper which splits an unreferenced folio

split_unref reads like "split and unref".

You probably want to call this something like "folio_split_frozen" ?

The very definition of "frozen" is "refcount = 0 ", so you can simplify 
the documentation.

Are the folios you want to pass in there completely unused (-> free) or 
might they still be in use (e.g., migration entries point at them during 
folio split)

So I am not sure yet if this should be "folio_split_frozen()" or 
"folio_split_freed()" or sth like that.

I'm not CCed on the other patches in the series or the cover letter, so 
I don't see the context.

You should describe in this patch here in which context the function is 
supposed to be used in later commits.

> (refcount == 0) into individual pages. Intended to be called on special
> pages (e.g., device-private, DAX, etc.) when returning the folio to the
> free page pool.
> 
> Cc: Balbir Singh <balbirs@...dia.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: David Hildenbrand <david@...nel.org>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Cc: Zi Yan <ziy@...dia.com>
> Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>
> Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>
> Cc: Nico Pache <npache@...hat.com>
> Cc: Ryan Roberts <ryan.roberts@....com>
> Cc: Dev Jain <dev.jain@....com>
> Cc: Barry Song <baohua@...nel.org>
> Cc: Lance Yang <lance.yang@...ux.dev>
> Cc: linux-mm@...ck.org
> Cc: linux-kernel@...r.kernel.org
> Suggested-by: Alistair Popple <apopple@...dia.com>
> Signed-off-by: Matthew Brost <matthew.brost@...el.com>
> Signed-off-by: Francois Dugast <francois.dugast@...el.com>
> ---
>   include/linux/huge_mm.h |  1 +
>   mm/huge_memory.c        | 39 +++++++++++++++++++++++++++++++++++++++
>   2 files changed, 40 insertions(+)
> 
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index a4d9f964dfde..18cb9728d8f1 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -369,6 +369,7 @@ enum split_type {
>   	SPLIT_TYPE_NON_UNIFORM,
>   };
>   
> +void folio_split_unref(struct folio *folio);
>   int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>   		unsigned int new_order);
>   int folio_split_unmapped(struct folio *folio, unsigned int new_order);
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 40cf59301c21..0eb9e6ad8639 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3580,6 +3580,45 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
>   		ClearPageCompound(&folio->page);
>   }
>   
> +/**
> + * folio_split_unref() - split an unreferenced folio (refcount == 0)
> + * @folio: the to-be-split folio
> + *
> + * Split an unreferenced folio (refcount == 0) into individual pages.
> + * Intended to be called on special pages (e.g., device-private, DAX, etc.)
> + * when returning the folio to the free page pool.
> + */
> +void folio_split_unref(struct folio *folio)
> +{
> +	struct dev_pagemap *pgmap = page_pgmap(&folio->page);
> +	int order, i;
> +
> +	folio->mapping = NULL;

It's unclear why you mess with the mapping. Usually, throughout a folio 
split, we populate the folio->mapping to all split folios.

-- 
Cheers

David