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

[David Hildenbrand <david@redhat.com>]

On 09.10.25 14:08, Christophe Leroy wrote:
> 
> 
> Le 09/10/2025 à 12:27, David Hildenbrand a écrit :
>> On 09.10.25 12:01, Christophe Leroy wrote:
>>>
>>>
>>> Le 09/10/2025 à 11:20, David Hildenbrand a écrit :
>>>> 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.
>>>
>>> In its name for sure, but the commit I mention says:
>>>
>>>        On systems without CONTIG_ALLOC activated but that support gigantic
>>> pages,
>>>        boottime reserved gigantic pages can not be freed at all.  This
>>> patch
>>>        simply enables the possibility to hand back those pages to memory
>>>        allocator.
>>
>> Right, I think it was a historical artifact.
>>
>>>
>>> And one of the hunks is:
>>>
>>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>>> index 7f7fbd8bd9d5b..7a1aa53d188d3 100644
>>> --- a/arch/arm64/Kconfig
>>> +++ b/arch/arm64/Kconfig
>>> @@ -19,7 +19,7 @@ config ARM64
>>>            select ARCH_HAS_FAST_MULTIPLIER
>>>            select ARCH_HAS_FORTIFY_SOURCE
>>>            select ARCH_HAS_GCOV_PROFILE_ALL
>>> -       select ARCH_HAS_GIGANTIC_PAGE if CONTIG_ALLOC
>>> +       select ARCH_HAS_GIGANTIC_PAGE
>>>            select ARCH_HAS_KCOV
>>>            select ARCH_HAS_KEEPINITRD
>>>            select ARCH_HAS_MEMBARRIER_SYNC_CORE
>>>
>>> So I understand from the commit message that it was possible at that
>>> time to have gigantic pages without ARCH_HAS_GIGANTIC_PAGE as long as
>>> you didn't have to be able to free them during runtime.
>>
>> Yes, I agree.
>>
>>>
>>>>
>>>> Can't we just select CONFIG_ARCH_HAS_GIGANTIC_PAGE for the relevant
>>>> hugetlb config that allows for *gigantic pages*.
>>>>
>>>
>>> We probably can, but I'd really like to understand history and how we
>>> ended up in the situation we are now.
>>> Because blind fixes often lead to more problems.
>>
>> Yes, let's figure out how to to it cleanly.
>>
>>>
>>> If I follow things correctly I see a helper gigantic_page_supported()
>>> added by commit 944d9fec8d7a ("hugetlb: add support for gigantic page
>>> allocation at runtime").
>>>
>>> And then commit 461a7184320a ("mm/hugetlb: introduce
>>> ARCH_HAS_GIGANTIC_PAGE") is added to wrap gigantic_page_supported()
>>>
>>> Then commit 4eb0716e868e ("hugetlb: allow to free gigantic pages
>>> regardless of the configuration") changed gigantic_page_supported() to
>>> gigantic_page_runtime_supported()
>>>
>>> So where are we now ?
>>
>> In
>>
>> commit fae7d834c43ccdb9fcecaf4d0f33145d884b3e5c
>> Author: Matthew Wilcox (Oracle) <willy@infradead.org>
>> Date:   Tue Feb 27 19:23:31 2024 +0000
>>
>>       mm: add __dump_folio()
>>
>>
>> We started assuming that a folio in the system (boottime, dynamic,
>> whatever)
>> has a maximum of MAX_FOLIO_NR_PAGES.
>>
>> Any other interpretation doesn't make any sense for MAX_FOLIO_NR_PAGES.
>>
>>
>> So we have two questions:
>>
>> 1) How to teach MAX_FOLIO_NR_PAGES that hugetlb supports gigantic pages
>>
>> 2) How do we handle CONFIG_ARCH_HAS_GIGANTIC_PAGE
>>
>>
>> We have the following options
>>
>> (A) Rename existing CONFIG_ARCH_HAS_GIGANTIC_PAGE to something else that is
>> clearer and add a new CONFIG_ARCH_HAS_GIGANTIC_PAGE.
>>
>> (B) Rename existing CONFIG_ARCH_HAS_GIGANTIC_PAGE -> to something else
>> that is
>> clearer and derive somehow else that hugetlb in that config supports
>> gigantic pages.
>>
>> (c) Just use CONFIG_ARCH_HAS_GIGANTIC_PAGE if hugetlb on an architecture
>> supports gigantic pages.
>>
>>
>> I don't quite see why an architecture should be able to opt in into
>> dynamically
>> allocating+freeing gigantic pages. That's just CONTIG_ALLOC magic and
>> not some
>> arch-specific thing IIRC.
>>
>>
>> Note that in mm/hugetlb.c it is
>>
>>       #ifdef CONFIG_ARCH_HAS_GIGANTIC_PAGE
>>       #ifdef CONFIG_CONTIG_ALLOC
>>
>> Meaning that at least the allocation side is guarded by CONTIG_ALLOC.
> 
> Yes but not the freeing since commit 4eb0716e868e ("hugetlb: allow to
> free gigantic pages regardless of the configuration")

Right, the freeing path is just always around as we no longer depend 
free_contig_range().

> 
>>
>> So I think (C) is just the right thing to do.
>>
>> diff --git a/fs/Kconfig b/fs/Kconfig
>> index 0bfdaecaa8775..12c11eb9279d3 100644
>> --- a/fs/Kconfig
>> +++ b/fs/Kconfig
>> @@ -283,6 +283,8 @@ config HUGETLB_PMD_PAGE_TABLE_SHARING
>>           def_bool HUGETLB_PAGE
>>           depends on ARCH_WANT_HUGE_PMD_SHARE && SPLIT_PMD_PTLOCKS
>>
>> +# An architecture must select this option if there is any mechanism
>> (esp. hugetlb)
>> +# could obtain gigantic folios.
>>    config ARCH_HAS_GIGANTIC_PAGE
>>           bool
>>
>>
> 
> I gave it a try. That's not enough, it fixes the problem for 64 Mbytes
> pages and 256 Mbytes pages, but not for 1 Gbytes pages.

Thanks!

> 
> Max folio is defined by PUD_ORDER, but PUD_SIZE is 256 Mbytes so we need
> to make MAX_FOLIO larger. Do we change it to P4D_ORDER or is it too much
> ? P4D_SIZE is 128 Gbytes

The exact size doesn't matter, we started with something that soundes 
reasonable.

I added the comment "There is no real limit on the folio size. We limit 
them to the maximum we currently expect (e.g., hugetlb, dax)."

We can set it to whatever we would expect for now.

-- 
Cheers

David / dhildenb