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

David Hildenbrand
2025-08-26T13:08:23Z

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