linux-kernel - Re: [PATCH v1 06/36] mm/page_alloc: reject unreasonable folio/compound page sizes in alloc_contig_range

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Message-ID: <4f6e66a1-1747-402e-8f1a-f6b7783fc2e5@redhat.com>
Date: Fri, 29 Aug 2025 15:09:45 +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>,
 SeongJae Park <sj@...nel.org>, 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,
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Subject: Re: [PATCH v1 06/36] mm/page_alloc: reject unreasonable
 folio/compound page sizes in alloc_contig_range_noprof()

> 
> It seems a bit arbitrary, like we open-code this (at risk of making a mistake)
> in some places but not others.

[...]

>>
>> One could argue that maybe one would want a order_to_pages() helper (that
>> could use BIT() internally), but I am certainly not someone that would
>> suggest that at this point ...  :)
> 
> I mean maybe.
> 
> Anyway as I said none of this is massively important, the open-coding here is
> correct, just seems silly.

Maybe we really want a ORDER_PAGES() and PAGES_ORDER().

But I mean, we also have PHYS_PFN() PFN_PHYS() and see how many "<< 
PAGE_SIZE" etc we are using all over the place.

> 
>>
>>>
>>>> +
>>>>    /*
>>>>     * 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 baead29b3e67b..426bc404b80cc 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)
> 
> Funny btw th
> 
>>>>    {
>>>> +	const unsigned int order = ilog2(end - start);
>>>>    	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;
>>>
>>> Possibly not worth it for a one off, but be nice to have this as a helper function, like:
>>>
>>> static bool is_valid_order(gfp_t gfp_mask, unsigned int order)
>>> {
>>> 	return !(gfp_mask & __GFP_COMP) || order <= MAX_FOLIO_ORDER;
>>> }
>>>
>>> Then makes this:
>>>
>>> 	if (WARN_ON_ONCE(!is_valid_order(gfp_mask, order)))
>>> 		return -EINVAL;
>>>
>>> Kinda self-documenting!
>>
>> I don't like it -- especially forwarding __GFP_COMP.
>>
>> is_valid_folio_order() to wrap the order check? Also not sure.
> 
> OK, it's not a big deal.
> 
> Can we have a comment explaining this though? As people might be confused
> as to why we check this here and not elsewhere.

I can add a comment.

-- 
Cheers

David / dhildenb