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

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Message-ID: <1db15a30-72d6-4045-8aa1-68bd8411b0ba@redhat.com>
Date: Thu, 9 Oct 2025 11:20:24 +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>,
 Brendan Jackman <jackmanb@...gle.com>, Christoph Lameter <cl@...two.org>,
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 iommu@...ts.linux.dev, io-uring@...r.kernel.org,
 Jason Gunthorpe <jgg@...dia.com>, Jens Axboe <axboe@...nel.dk>,
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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 11:16, Christophe Leroy wrote:
> 
> 
> Le 09/10/2025 à 10:14, David Hildenbrand a écrit :
>> 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.
> 
> I'm trying to dig into history and when looking at commit 4eb0716e868e
> ("hugetlb: allow to free gigantic pages regardless of the
> configuration") I understand that CONFIG_ARCH_HAS_GIGANTIC_PAGE is
> needed to be able to allocate gigantic pages at runtime. It is not
> needed to reserve gigantic pages at boottime.
> 
> What am I missing ?

That CONFIG_ARCH_HAS_GIGANTIC_PAGE has nothing runtime-specific in its name.

Can't we just select CONFIG_ARCH_HAS_GIGANTIC_PAGE for the relevant 
hugetlb config that allows for *gigantic pages*.

-- 
Cheers

David / dhildenb