Re: [PATCH v1 11/36] mm: limit folio/compound page sizes in problematic kernel configs

On Fri, Aug 29, 2025 at 01:57:22PM +0200, David Hildenbrand wrote:
> On 28.08.25 17:10, Lorenzo Stoakes wrote:
> > On Thu, Aug 28, 2025 at 12:01:15AM +0200, David Hildenbrand wrote:
> > > Let's limit the maximum folio size in problematic kernel config where
> > > the memmap is allocated per memory section (SPARSEMEM without
> > > SPARSEMEM_VMEMMAP) to a single memory section.
> > >
> > > Currently, only a single architectures supports ARCH_HAS_GIGANTIC_PAGE
> > > but not SPARSEMEM_VMEMMAP: sh.
> > >
> > > Fortunately, the biggest hugetlb size sh supports is 64 MiB
> > > (HUGETLB_PAGE_SIZE_64MB) and the section size is at least 64 MiB
> > > (SECTION_SIZE_BITS == 26), so their use case is not degraded.
> > >
> > > As folios and memory sections are naturally aligned to their order-2 size
> > > in memory, consequently a single folio can no longer span multiple memory
> > > sections on these problematic kernel configs.
> > >
> > > nth_page() is no longer required when operating within a single compound
> > > page / folio.
> > >
> > > Reviewed-by: Zi Yan <[email protected]>
> > > Acked-by: Mike Rapoport (Microsoft) <[email protected]>
> > > Signed-off-by: David Hildenbrand <[email protected]>
> >
> > Realy great comments, like this!
> >
> > I wonder if we could have this be part of the first patch where you fiddle
> > with MAX_FOLIO_ORDER etc. but not a big deal.
>
> I think it belongs into this patch where we actually impose the
> restrictions.

Sure it's not a big deal.

>
> [...]
>
> > > +/*
> > > + * Only pages within a single memory section are guaranteed to be
> > > + * contiguous. By limiting folios to a single memory section, all folio
> > > + * pages are guaranteed to be contiguous.
> > > + */
> > > +#define MAX_FOLIO_ORDER		PFN_SECTION_SHIFT
> >
> > Hmmm, was this implicit before somehow? I mean surely by the fact as you say
> > that physical contiguity would not otherwise be guaranteed :))
>
> Well, my patches until this point made sure that any attempt to use a larger
> folio would fail in a way that we could spot now if there is any offender.

Ack yeah.

>
> That is why before this change, nth_page() was required within a folio.
>
> Hope that clarifies it, thanks!

Yes thanks! :)

>
> --
> Cheers
>
> David / dhildenb
>

Cheers, Lorenzo