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: <D9725DC7-8B7E-460D-8859-09F78312DC9C@nvidia.com>
Date: Fri, 21 Nov 2025 12:24:50 -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 21 Nov 2025, at 12:09, David Hildenbrand (Red Hat) wrote:

>>>
>>> BTW, I wonder if the is_huge_zero_folio() check should go into folio_split_supported() and just return in -EINVAL. (we shouldn't really trigger that). Similarly we could add a hugetlb sanity check.
>>
>> Yeah, is_huge_zero_folio() should return -EINVAL not -EBUSY, except
>> the case the split happens before a process writes 0 to a zero large folio
>> and gets a new writable large folio, in which we can kinda say it looks like
>> -EBUSY. But it is still a stretch.
>
> I see what you mean, but I think this has less to do with actual races. SO yeah, -EINVAL is likely the tight thing.
>
Sure. Will move it and use -EINVAL.

>>
>> Ack on adding hugetlb sanity check.
>>
>> OK, just to reiterate my above idea on renaming folio_split_supported().
>> Are you OK with renaming it to folio_split_check(), so that returning -EBUSY
>> and -EINVAL looks more reasonable? The benefit is that we no longer need
>> to worry about we need to always do folio->mapping check before
>> folio_split_supported(). (In addition, I would rename can_split_folio()
>> to folio_split_refcount_check() for clarification)
> I guess having some function that tells you "I performed all checks I could without taking locks/references (like anon_vma) and starting with the real magic" is what you have in mind.

Yes.

>
> For these we don't have to prefix with "folio_split" if it sounds weird.
>
> folio_check_splittable() ?

Sounds good to me.

>
> Regarding can_split_folio(), I was wondering whether we can just get rid of it and use folio_expect_ref_count() instead?
>
> For the two callers that need extra_pins, we could just have something simple helper in huge_memory.c like
>
> /* Number of folio references from the pagecache or the swapcache. */
> unsigned int folio_cache_references(const struct folio *folio)
> {
> 	if (folio_test_anon(folio) && !folio_test_swapcache(folio))
> 		return 0;
> 	return folio_nr_pages(folio);
> }

OK, I will give this a try in a separate patch in an updated version of this
series.

Thank you for the feedback.

Best Regards,
Yan, Zi