linux-kernel - Re: [PATCH RFC 06/35] mm/page_alloc: reject unreasonable folio/compound page sizes in alloc_contig_range

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Message-ID: <a04d8499-85ad-40b4-8173-dcc81a5a71bf@nvidia.com>
Date: Thu, 9 Oct 2025 21:25:42 +1100
From: Balbir Singh <balbirs@...dia.com>
To: David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org
Cc: 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,
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 Jason Gunthorpe <jgg@...dia.com>, Jens Axboe <axboe@...nel.dk>,
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 kasan-dev@...glegroups.com, kvm@...r.kernel.org,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
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 Vlastimil Babka <vbabka@...e.cz>, wireguard@...ts.zx2c4.com, x86@...nel.org,
 Zi Yan <ziy@...dia.com>
Subject: Re: [PATCH RFC 06/35] mm/page_alloc: reject unreasonable
 folio/compound page sizes in alloc_contig_range_noprof()

On 10/9/25 17:12, David Hildenbrand wrote:
> On 09.10.25 06:21, Balbir Singh wrote:
>> On 8/22/25 06:06, David Hildenbrand wrote:
>>> Let's reject them early, which in turn makes folio_alloc_gigantic() reject
>>> them properly.
>>>
>>> To avoid converting from order to nr_pages, let's just add MAX_FOLIO_ORDER
>>> and calculate MAX_FOLIO_NR_PAGES based on that.
>>>
>>> Signed-off-by: David Hildenbrand <david@...hat.com>
>>> ---
>>>   include/linux/mm.h | 6 ++++--
>>>   mm/page_alloc.c    | 5 ++++-
>>>   2 files changed, 8 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>>> index 00c8a54127d37..77737cbf2216a 100644
>>> --- a/include/linux/mm.h
>>> +++ b/include/linux/mm.h
>>> @@ -2055,11 +2055,13 @@ static inline long folio_nr_pages(const struct folio *folio)
>>>     /* Only hugetlbfs can allocate folios larger than MAX_ORDER */
>>>   #ifdef CONFIG_ARCH_HAS_GIGANTIC_PAGE
>>> -#define MAX_FOLIO_NR_PAGES    (1UL << PUD_ORDER)
>>> +#define MAX_FOLIO_ORDER        PUD_ORDER
>>
>> Do we need to check for CONTIG_ALLOC as well with CONFIG_ARCH_HAS_GIGANTIC_PAGE?
>>
> 
> I don't think so, can you elaborate?
> 

The only way to allocate a gigantic page is to use CMA, IIRC, which is covered by CONTIG_ALLOC

>>>   #else
>>> -#define MAX_FOLIO_NR_PAGES    MAX_ORDER_NR_PAGES
>>> +#define MAX_FOLIO_ORDER        MAX_PAGE_ORDER
>>>   #endif
>>>   +#define MAX_FOLIO_NR_PAGES    (1UL << MAX_FOLIO_ORDER)
>>> +
>>>   /*
>>>    * compound_nr() returns the number of pages in this potentially compound
>>>    * page.  compound_nr() can be called on a tail page, and is defined to
>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>> index ca9e6b9633f79..1e6ae4c395b30 100644
>>> --- a/mm/page_alloc.c
>>> +++ b/mm/page_alloc.c
>>> @@ -6833,6 +6833,7 @@ static int __alloc_contig_verify_gfp_mask(gfp_t gfp_mask, gfp_t *gfp_cc_mask)
>>>   int alloc_contig_range_noprof(unsigned long start, unsigned long end,
>>>                     acr_flags_t alloc_flags, gfp_t gfp_mask)
>>>   {
>>> +    const unsigned int order = ilog2(end - start);
>>
>> Do we need a VM_WARN_ON(end < start)?
> 
> I don't think so.
> 

end - start being < 0, completely breaks ilog2. But we would error out because ilog2 > MAX_FOLIO_ORDER, so we should fine

>>
>>>       unsigned long outer_start, outer_end;
>>>       int ret = 0;
>>>   @@ -6850,6 +6851,9 @@ int alloc_contig_range_noprof(unsigned long start, unsigned long end,
>>>                           PB_ISOLATE_MODE_CMA_ALLOC :
>>>                           PB_ISOLATE_MODE_OTHER;
>>>   +    if (WARN_ON_ONCE((gfp_mask & __GFP_COMP) && order > MAX_FOLIO_ORDER))
>>> +        return -EINVAL;
>>> +
>>>       gfp_mask = current_gfp_context(gfp_mask);
>>>       if (__alloc_contig_verify_gfp_mask(gfp_mask, (gfp_t *)&cc.gfp_mask))
>>>           return -EINVAL;
>>> @@ -6947,7 +6951,6 @@ int alloc_contig_range_noprof(unsigned long start, unsigned long end,
>>>               free_contig_range(end, outer_end - end);
>>>       } else if (start == outer_start && end == outer_end && is_power_of_2(end - start)) {
>>>           struct page *head = pfn_to_page(start);
>>> -        int order = ilog2(end - start);
>>>             check_new_pages(head, order);
>>>           prep_new_page(head, order, gfp_mask, 0);
>>
>> Acked-by: Balbir Singh <balbirs@...dia.com>
> 
> Thanks for the review, but note that this is already upstream.
> 

Sorry, this showed up in my updated mm thread and I ended up reviewing it, please ignore if it's upstream

Balbir