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

[David Hildenbrand (Red Hat)]

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,

--
Cheers

David