linux-kernel - Re: [RFC PATCH 1/3] mm/huge_memory: prevent NULL pointer dereference in try_folio_split_to

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Message-ID: <FCFED9E9-56A9-4260-8B42-D0BCFC83C506@nvidia.com>
Date: Thu, 20 Nov 2025 09:41:53 -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 1/3] mm/huge_memory: prevent NULL pointer dereference
 in try_folio_split_to_order()

On 20 Nov 2025, at 4:25, David Hildenbrand (Red Hat) wrote:

> On 11/20/25 04:59, Zi Yan wrote:
>> folio_split_supported() used in try_folio_split_to_order() requires
>> folio->mapping to be non NULL, but current try_folio_split_to_order() does
>> not check it. Add the check to prevent NULL pointer dereference.
>>
>> There is no issue in the current code, since try_folio_split_to_order() is
>> only used in truncate_inode_partial_folio(), where folio->mapping is not
>> NULL.
>>
>> Signed-off-by: Zi Yan <ziy@...dia.com>
>> ---
>>   include/linux/huge_mm.h | 7 +++++++
>>   1 file changed, 7 insertions(+)
>>
>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>> index 1d439de1ca2c..0d55354e3a34 100644
>> --- a/include/linux/huge_mm.h
>> +++ b/include/linux/huge_mm.h
>> @@ -407,6 +407,13 @@ static inline int split_huge_page_to_order(struct page *page, unsigned int new_o
>>   static inline int try_folio_split_to_order(struct folio *folio,
>>   		struct page *page, unsigned int new_order)
>>   {
>> +	/*
>> +	 * Folios that just got truncated cannot get split. Signal to the
>> +	 * caller that there was a race.
>> +	 */
>> +	if (!folio_test_anon(folio) && !folio->mapping)
>> +		return -EBUSY;
>> +
>>   	if (!folio_split_supported(folio, new_order, SPLIT_TYPE_NON_UNIFORM, /* warns= */ false))
>>   		return split_huge_page_to_order(&folio->page, new_order);
>>   	return folio_split(folio, new_order, page, NULL);
>
> I guess we'll take the one from Wei
>
> https://lkml.kernel.org/r/20251119235302.24773-1-richard.weiyang@gmail.com
>
> right?

This is different. Wei’s fix is to __folio_split(), but mine is to
try_folio_split_to_order(). Both call folio_split_supported(), thus
both need the folio->mapping check.

That is also my question in the cover letter on whether we should
move folio->mapping check to folio_split_supported() and return
error code instead of bool. Otherwise, any folio_split_supported()
caller needs to check folio->mapping.

Best Regards,
Yan, Zi