netdev - Re: [PATCH RFC 21/35] mm/cma: refuse handing out non-contiguous page ranges

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Message-ID: <ecc599ee-4175-4356-ab66-1d76a75f44f7@redhat.com>
Date: Tue, 26 Aug 2025 15:08:08 +0200
From: David Hildenbrand <david@...hat.com>
To: Alexandru Elisei <alexandru.elisei@....com>
Cc: linux-kernel@...r.kernel.org, Alexander Potapenko <glider@...gle.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Brendan Jackman <jackmanb@...gle.com>, Christoph Lameter <cl@...two.org>,
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 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>, linux-arm-kernel@...s.com,
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 Zi Yan <ziy@...dia.com>
Subject: Re: [PATCH RFC 21/35] mm/cma: refuse handing out non-contiguous page
 ranges

On 26.08.25 15:03, Alexandru Elisei wrote:
> Hi David,
> 
> On Tue, Aug 26, 2025 at 01:04:33PM +0200, David Hildenbrand wrote:
> ..
>>> Just so I can better understand the problem being fixed, I guess you can have
>>> two consecutive pfns with non-consecutive associated struct page if you have two
>>> adjacent memory sections spanning the same physical memory region, is that
>>> correct?
>>
>> Exactly. Essentially on SPARSEMEM without SPARSEMEM_VMEMMAP it is not
>> guaranteed that
>>
>> 	pfn_to_page(pfn + 1) == pfn_to_page(pfn) + 1
>>
>> when we cross memory section boundaries.
>>
>> It can be the case for early boot memory if we allocated consecutive areas
>> from memblock when allocating the memmap (struct pages) per memory section,
>> but it's not guaranteed.
> 
> Thank you for the explanation, but I'm a bit confused by the last paragraph. I
> think what you're saying is that we can also have the reverse problem, where
> consecutive struct page * represent non-consecutive pfns, because memmap
> allocations happened to return consecutive virtual addresses, is that right?

Exactly, that's something we have to deal with elsewhere [1]. For this 
code, it's not a problem because we always allocate a contiguous PFN range.

> 
> If that's correct, I don't think that's the case for CMA, which deals out
> contiguous physical memory. Or were you just trying to explain the other side of
> the problem, and I'm just overthinking it?

The latter :)

[1] https://lkml.kernel.org/r/20250814064714.56485-2-lizhe.67@bytedance.com

-- 
Cheers

David / dhildenb