Re: [PATCH 2/2] mm: convert do_set_pmd() to take a folio

[Matthew Wilcox]

On Thu, May 08, 2025 at 09:36:02AM +0200, David Hildenbrand wrote:
> > > Which raises an interesting question: I assume in the future, when we have a 4 MiB folio on x86-64 that is *misaligned* in VA space regarding PMDs (e.g., aligned to 1 MiB but not 2 MiB), we could still allow to use a PMD for the middle part.
> > 
> > It might not be possible if the folio comes from buddy allocator due to how
> > buddy allocator merges a PFN with its buddy (see __find_buddy_pfn() in mm/internal.h).
> > A 4MB folio will always be two 2MB-aligned parts. In addition, VA and PA need
> > to have the same lower 9+12 bits for a PMD mapping. So PMD mappings for
> > a 4MB folio would always be two PMDs. Let me know if I miss anything.
> 
> PA is clear. But is mis-alignment in VA space impossible on all
> architectures? I certainly remember it being impossible on x86-64 and s390x
> (remaining PMD entry bits used for something else).

Yeah, that's not possible; the PMD is always PMD-aligned.