linux-kernel - Re: [RFC PATCH 2/3] mm/huge_memory: add kernel-doc for folio_split

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Message-ID: <b051bc10-cf68-4e1b-847d-a1323552b963@kernel.org>
Date: Thu, 20 Nov 2025 10:27:20 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Zi Yan <ziy@...dia.com>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 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>,
 Miaohe Lin <linmiaohe@...wei.com>, Naoya Horiguchi
 <nao.horiguchi@...il.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 2/3] mm/huge_memory: add kernel-doc for
 folio_split_supported()

On 11/20/25 04:59, Zi Yan wrote:
> It clarifies that folio_split_supported() does not check folio->mapping and
> can dereference it.
> 
> Signed-off-by: Zi Yan <ziy@...dia.com>
> ---
>   mm/huge_memory.c | 17 +++++++++++++++++
>   1 file changed, 17 insertions(+)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index efea42d68157..15e555f1b85d 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3688,6 +3688,23 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
>   	return 0;
>   }
>   
> +/**
> + * folio_split_supported() - check if a folio can be split to a given order
> + * @folio: folio to be split
> + * @new_order: the smallest order of the after split folios (since buddy
> + *             allocator like split generates folios with orders from @folio's
> + *             order - 1 to new_order).
> + * @split_type: uniform or non-uniform split
> + * @warns: whether gives warnings or not for the checks in the function
> + *
> + * folio_split_supported() checks if @folio can be split to @new_order using
> + * @split_type method.
> + *
> + * Context: Caller must make sure folio->mapping is not NULL, since the
> + *          function does not check it and can dereference folio->mapping

Only for anon folios. Also, I would drop the detail about dereference.

I guess we really need the folio lock to prevent concurrent truncation.

Maybe something like:

"The folio must be locked. For non-anon folios, the caller must make 
sure that folio->mapping is not NULL (e.g., not truncated)."

-- 
Cheers

David