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

Christophe Leroy
2025-10-09T08:22:00Z

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

Christophe