Re: [PATCHv6 05/17] riscv/mm: Align vmemmap to maximal folio size

David Hildenbrand (Arm)
2026-02-05T13:54:59Z

On 2/5/26 14:50, Kiryl Shutsemau wrote:
> On Wed, Feb 04, 2026 at 05:50:23PM +0100, David Hildenbrand (arm) wrote:
>> On 2/2/26 16:56, Kiryl Shutsemau wrote:
>>> The upcoming change to the HugeTLB vmemmap optimization (HVO) requires
>>> struct pages of the head page to be naturally aligned with regard to the
>>> folio size.
>>>
>>> Align vmemmap to MAX_FOLIO_NR_PAGES.
>>
>> I think neither that statement nor the one in the patch description is
>> correct?
>>
>> "MAX_FOLIO_NR_PAGES * sizeof(struct page)" is neither the maximum folio size
>> nor MAX_FOLIO_NR_PAGES.
>>
>> It's the size of the memmap that a large folio could span at maximum.
>>
>>
>> Assuming we have a 16 GiB folio, the calculation would give us
>>
>> 4194304 * sizeof(struct page)
>>
>> Which could be something like (assuming 80 bytes)
>>
>> 335544320
>>
>> -> not even a power of 2, weird? (for HVO you wouldn't care as HVO would be
>> disabled, but that aliment is super weird?)
>>
>>
>> Assuming 64 bytes, it would be a power of two (as 64 is a power of two).
>>
>> 268435456 (1<< 28)
>>
>>
>> Which makes me wonder whether there is a way to avoid sizeof(struct page)
>> here completely.
>
> I don't think we can. See the other thread.

Agreed. You could only go for something larger (like PAGE_SIZE).

>
> What about using roundup_pow_of_two(sizeof(struct page)) here.

Better I think.

>
>> Or limit the alignment to the case where HVO is actually active and
>> sizeof(struct page) makes any sense?
>
> The annoying part of HVO is that it is unknown at compile-time if it
> will be used. You can compile kernel with HVO that will no be activated
> due to non-power-of-2 sizeof(struct page) because of a debug config option.
Ah, and now I remember that sizeof cannot be used in macros, damnit.

--
Cheers,

David