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: <010f0f19-8b87-4537-8c0b-bc8f9263aab4@nvidia.com>
Date: Thu, 20 Nov 2025 15:28:33 +1100
From: Balbir Singh <balbirs@...dia.com>
To: Zi Yan <ziy@...dia.com>, David Hildenbrand <david@...nel.org>,
 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 1/3] mm/huge_memory: prevent NULL pointer dereference
 in try_folio_split_to_order()

On 11/20/25 14:59, 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.
> 

Just reading through the description does not clarify one thing

What is the race between just truncated and trying to split them - is there a common lock 
that needs to be held? Is it the subsequent call in truncate_inode_partial_folio()
that causes the race?

IOW, if a folio is not anonymous and does not have a mapping, how is it
being passed to this function?

> 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);