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

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Message-ID: <d3fc12d4-0b59-4b1f-bb5c-13189a01e13d@redhat.com>
Date: Thu, 9 Oct 2025 09:22:14 +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>,
 Dennis Zhou <dennis@...nel.org>, Dmitry Vyukov <dvyukov@...gle.com>,
 dri-devel@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
 iommu@...ts.linux.dev, io-uring@...r.kernel.org,
 Jason Gunthorpe <jgg@...dia.com>, Jens Axboe <axboe@...nel.dk>,
 Johannes Weiner <hannes@...xchg.org>, John Hubbard <jhubbard@...dia.com>,
 kasan-dev@...glegroups.com, kvm@...r.kernel.org,
 Linus Torvalds <torvalds@...ux-foundation.org>, linux-arm-kernel@...s.com,
 linux-arm-kernel@...ts.infradead.org, linux-crypto@...r.kernel.org,
 linux-ide@...r.kernel.org, linux-kselftest@...r.kernel.org,
 linux-mips@...r.kernel.org, linux-mmc@...r.kernel.org, linux-mm@...ck.org,
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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 09:14, Christophe Leroy wrote:
> Hi David,
> 
> Le 01/09/2025 à 17:03, David Hildenbrand a écrit :
>> Let's check that no hstate that corresponds to an unreasonable folio size
>> is registered by an architecture. If we were to succeed registering, we
>> could later try allocating an unsupported gigantic folio size.
>>
>> Further, let's add a BUILD_BUG_ON() for checking that HUGETLB_PAGE_ORDER
>> is sane at build time. As HUGETLB_PAGE_ORDER is dynamic on powerpc, we have
>> to use a BUILD_BUG_ON_INVALID() to make it compile.
>>
>> No existing kernel configuration should be able to trigger this check:
>> either SPARSEMEM without SPARSEMEM_VMEMMAP cannot be configured or
>> gigantic folios will not exceed a memory section (the case on sparse).
>>
>> Reviewed-by: Zi Yan <ziy@...dia.com>
>> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>> Reviewed-by: Liam R. Howlett <Liam.Howlett@...cle.com>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
> 
> I get following warning on powerpc with linus tree, bisected to commit
> 7b4f21f5e038 ("mm/hugetlb: check for unreasonable folio sizes when
> registering hstate")

Do you have the kernel config around? Is it 32bit?

That would be helpful.

[...]

>> ---
>>    mm/hugetlb.c | 2 ++
>>    1 file changed, 2 insertions(+)
>>
>> 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).

-- 
Cheers

David / dhildenb