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 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.

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
  


-- 
Cheers

David / dhildenb