Re: [PATCHv2 02/14] mm/sparse: Check memmap alignment

[Muchun Song]

> On Dec 22, 2025, at 22:18, David Hildenbrand (Red Hat) <david@xxxxxxxxxx> wrote:
> 
> On 12/22/25 15:02, Kiryl Shutsemau wrote:
>>> On Mon, Dec 22, 2025 at 04:34:40PM +0800, Muchun Song wrote:
>>> 
>>> 
>>> On 2025/12/18 23:09, Kiryl Shutsemau wrote:
>>>> The upcoming changes in compound_head() require memmap to be naturally
>>>> aligned to the maximum folio size.
>>>> 
>>>> Add a warning if it is not.
>>>> 
>>>> A warning is sufficient as MAX_FOLIO_ORDER is very rarely used, so the
>>>> kernel is still likely to be functional if this strict check fails.
>>> 
>>> Different architectures default to 2 MB alignment (mainly to
>>> enable huge mappings), which only accommodates folios up to
>>> 128 MB. Yet 1 GB huge pages are still fairly common, so
>>> validating 16 GB (MAX_FOLIO_SIZE) alignment seems likely to
>>> miss the most frequent case.
>> I don't follow. 16 GB check is more strict that anything smaller.
>> How can it miss the most frequent case?
>>> I’m concerned that this might plant a hidden time bomb: it
>>> could detonate at any moment in later code, silently triggering
>>> memory corruption or similar failures. Therefore, I don’t
>>> think a WARNING is a good choice.
>> We can upgrade it BUG_ON(), but I want to understand your logic here
>> first.
> 
> Definitely no BUG_ON(). I would assume this is something we would find early during testing, so even a VM_WARN_ON_ONCE() should be good enough?
> 
> This smells like a possible problem, though, as soon as some architecture wants to increase the folio size. What would be the expected step to ensure the alignment is done properly?
> 
> But OTOH, as I raised Willy's work will make all of that here obsolete either way, so maybe not worth worrying about that case too much,

Hi David,

I hope you're doing well. I must admit I have limited knowledge of Willy's work, and I was wondering if you might be kind enough to share any publicly available links where I could learn more about the future direction of this project. I would be truly grateful for your guidance.
Thank you very much in advance.

Best regards,

> 
> --
> Cheers
> 
> David