linux-kernel - Re: [PATCH v1 10/36] mm: sanity-check maximum folio size in folio_set

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Message-ID: <a9b2b570-dc81-43dd-b2f3-a82a8de37705@redhat.com>
Date: Fri, 29 Aug 2025 12:10:30 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: linux-kernel@...r.kernel.org, Zi Yan <ziy@...dia.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,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>, linux-arm-kernel@...s.com,
 linux-arm-kernel@...ts.infradead.org, linux-crypto@...r.kernel.org,
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 linux-scsi@...r.kernel.org, Marco Elver <elver@...gle.com>,
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Subject: Re: [PATCH v1 10/36] mm: sanity-check maximum folio size in
 folio_set_order()

On 28.08.25 17:00, Lorenzo Stoakes wrote:
> On Thu, Aug 28, 2025 at 12:01:14AM +0200, David Hildenbrand wrote:
>> Let's sanity-check in folio_set_order() whether we would be trying to
>> create a folio with an order that would make it exceed MAX_FOLIO_ORDER.
>>
>> This will enable the check whenever a folio/compound page is initialized
>> through prepare_compound_head() / prepare_compound_page().
> 
> NIT: with CONFIG_DEBUG_VM set :)

Yes, will add that.

> 
>>
>> Reviewed-by: Zi Yan <ziy@...dia.com>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
> 
> LGTM (apart from nit below), so:
> 
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> 
>> ---
>>   mm/internal.h | 1 +
>>   1 file changed, 1 insertion(+)
>>
>> diff --git a/mm/internal.h b/mm/internal.h
>> index 45da9ff5694f6..9b0129531d004 100644
>> --- a/mm/internal.h
>> +++ b/mm/internal.h
>> @@ -755,6 +755,7 @@ static inline void folio_set_order(struct folio *folio, unsigned int order)
>>   {
>>   	if (WARN_ON_ONCE(!order || !folio_test_large(folio)))
>>   		return;
>> +	VM_WARN_ON_ONCE(order > MAX_FOLIO_ORDER);
> 
> Given we have 'full-fat' WARN_ON*()'s above, maybe worth making this one too?

The idea is that if you reach this point here, previous such checks I 
added failed. So this is the safety net, and for that VM_WARN_ON_ONCE() 
is sufficient.

I think we should rather convert the WARN_ON_ONCE to VM_WARN_ON_ONCE() 
at some point, because no sane code should ever trigger that.

-- 
Cheers

David / dhildenb