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

David Hildenbrand
2025-10-09T08:14:12Z

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