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

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Message-ID: <9D88B1A6-1D00-4770-9B66-A9A0F1B6AC15@nvidia.com>
Date: Thu, 20 Nov 2025 09:48:31 -0500
From: Zi Yan <ziy@...dia.com>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 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 20 Nov 2025, at 4:27, David Hildenbrand (Red Hat) wrote:

> 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.

OK.

>
> 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)."

Sure. Do you think it is worth adding VM_WARN_ONCE_ON(!folio_test_locked);
and VM_WARN_ONCE_ON(!folio->mapping); ?

Best Regards,
Yan, Zi