linux-kernel - Re: (bisected) [PATCH v2 08/37] mm/hugetlb: check for unreasonable folio sizes when registering hstate

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Message-ID: <9361c75a-ab37-4d7f-8680-9833430d93d4@redhat.com>
Date: Thu, 9 Oct 2025 10:14:02 +0200
From: David Hildenbrand <david@...hat.com>
To: Christophe Leroy <christophe.leroy@...roup.eu>,
 linux-kernel@...r.kernel.org
Cc: Zi Yan <ziy@...dia.com>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Alexander Potapenko <glider@...gle.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
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 "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: (bisected) [PATCH v2 08/37] mm/hugetlb: check for unreasonable
 folio sizes when registering hstate

On 09.10.25 10:04, Christophe Leroy wrote:
> 
> 
> Le 09/10/2025 à 09:22, David Hildenbrand a écrit :
>> On 09.10.25 09:14, Christophe Leroy wrote:
>>> Hi David,
>>>
>>> Le 01/09/2025 à 17:03, David Hildenbrand a écrit :
>>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>>>> index 1e777cc51ad04..d3542e92a712e 100644
>>>> --- a/mm/hugetlb.c
>>>> +++ b/mm/hugetlb.c
>>>> @@ -4657,6 +4657,7 @@ static int __init hugetlb_init(void)
>>>>         BUILD_BUG_ON(sizeof_field(struct page, private) * BITS_PER_BYTE <
>>>>                 __NR_HPAGEFLAGS);
>>>> +    BUILD_BUG_ON_INVALID(HUGETLB_PAGE_ORDER > MAX_FOLIO_ORDER);
>>>>         if (!hugepages_supported()) {
>>>>             if (hugetlb_max_hstate || default_hstate_max_huge_pages)
>>>> @@ -4740,6 +4741,7 @@ void __init hugetlb_add_hstate(unsigned int order)
>>>>         }
>>>>         BUG_ON(hugetlb_max_hstate >= HUGE_MAX_HSTATE);
>>>>         BUG_ON(order < order_base_2(__NR_USED_SUBPAGE));
>>>> +    WARN_ON(order > MAX_FOLIO_ORDER);
>>>>         h = &hstates[hugetlb_max_hstate++];
>>>>         __mutex_init(&h->resize_lock, "resize mutex", &h->resize_key);
>>>>         h->order = order;
>>
>> We end up registering hugetlb folios that are bigger than
>> MAX_FOLIO_ORDER. So we have to figure out how a config can trigger that
>> (and if we have to support that).
>>
> 
> MAX_FOLIO_ORDER is defined as:
> 
> #ifdef CONFIG_ARCH_HAS_GIGANTIC_PAGE
> #define MAX_FOLIO_ORDER		PUD_ORDER
> #else
> #define MAX_FOLIO_ORDER		MAX_PAGE_ORDER
> #endif
> 
> MAX_PAGE_ORDER is the limit for dynamic creation of hugepages via
> /sys/kernel/mm/hugepages/ but bigger pages can be created at boottime
> with kernel boot parameters without CONFIG_ARCH_HAS_GIGANTIC_PAGE:
> 
>     hugepagesz=64m hugepages=1 hugepagesz=256m hugepages=1
> 
> Gives:
> 
> HugeTLB: registered 1.00 GiB page size, pre-allocated 0 pages
> HugeTLB: 0 KiB vmemmap can be freed for a 1.00 GiB page
> HugeTLB: registered 64.0 MiB page size, pre-allocated 1 pages
> HugeTLB: 0 KiB vmemmap can be freed for a 64.0 MiB page
> HugeTLB: registered 256 MiB page size, pre-allocated 1 pages
> HugeTLB: 0 KiB vmemmap can be freed for a 256 MiB page
> HugeTLB: registered 4.00 MiB page size, pre-allocated 0 pages
> HugeTLB: 0 KiB vmemmap can be freed for a 4.00 MiB page
> HugeTLB: registered 16.0 MiB page size, pre-allocated 0 pages
> HugeTLB: 0 KiB vmemmap can be freed for a 16.0 MiB page

I think it's a violation of CONFIG_ARCH_HAS_GIGANTIC_PAGE. The existing 
folio_dump() code would not handle it correctly as well.

See how snapshot_page() uses MAX_FOLIO_NR_PAGES.

-- 
Cheers

David / dhildenb