Re: [PATCH] mm/sparse: fix BUILD_BUG_ON check for section map alignment

David Hildenbrand (Arm)
2026-04-01T07:08:27Z

On 4/1/26 06:01, Muchun Song wrote:
>
>
>> On Apr 1, 2026, at 10:57, Muchun Song <muchun.song@linux.dev> wrote:
>>
>>
>>
>>>
>>>
>>> If that would trigger, wouldn't the memmap of a memory section be
>>> smaller than a single page?
>>
>> I don't think a memory section can be smaller than a page, because
>> PFN_SECTION_SHIFT is defined as follows:
>>
>> #define PFN_SECTION_SHIFT (SECTION_SIZE_BITS - PAGE_SHIFT)
>>
>> Therefore, PFN_SECTION_SHIFT must be greater than PAGE_SHIFT. On powerpc,
>> PFN_SECTION_SHIFT is 6, PAGE_SHIFT is 18 (the worst combination).
>>
>> Sorry, but I didn't understand what your concern is. Could you elaborate
>> a bit more?
>
> Sorry, I misread it earlier — I thought it was about the memory
> section size, but it's actually about the memmap size. Let me respond
> again to your question. On the powerpc architecture, when PFN_SECTION_SHIFT
> is 6 and PAGE_SHIFT is 18, it does seem that the memmap of a memory
> section would be smaller than a single page.

Right, and I am saying we don't care about that and do not support it.

For example, most vmemmap code I am aware of relies on a single section
covering full pages.

So I don't think this patch here adds any value, except faking that we
might be supporting something we don't?

--
Cheers,

David