Re: [RFC PATCH 01/57] mm: add pte_folio()

[Lorenzo Stoakes (ARM)]

On Tue, Aug 18, 2026 at 07:09:48PM +0200, David Hildenbrand (Arm) wrote:
> On 8/17/26 00:45, Kiryl Shutsemau wrote:
> > From: "Kiryl Shutsemau (Meta)" <kas@xxxxxxxxxx>
> >
> > Callers that want the folio behind a present PTE spell it out as
> > page_folio(pte_page(pte)).
> >
> > Add pte_folio() as the folio companion to pte_page(), and convert the
> > callers in fs/proc/task_mmu.c and mm/hugetlb.c.
> >
> > Preparation for the anonymous collapse engine, which reads the folio
> > behind a PTE in several places.
>
> [...]
>
> >
> > +/**
> > + * pte_folio - Return the folio mapped by a present PTE.
> > + * @pte: A present page table entry.
> > + *
> > + * The folio companion to pte_page(); only meaningful for a present PTE
> > + * that maps a struct-page-backed folio.
> > + *
> > + * Return: The folio containing the page @pte maps.
> > + */
> > +static inline struct folio *pte_folio(pte_t pte)
> > +{
> > +	return page_folio(pte_page(pte));
> > +}
>
> There is a reason why most code doesn't need that: because they should be using
> vm_normal_page() / vm_normal_folio(), or need the exact page and handle special
> ptes differently (see gup.c that uses pte_page()).
>
> And other code that uses pte_page() doesn't really operate on folios AFAIKs.
>
> That's also why you are only touching hugetlb code here.
>
> IOW, there must be a pretty good reason for us to add a non-hugetlb helper when
> that looks like a good fit for common code when it's really only hugetlb that
> does weird things (and doesn't need the exact page!).
>
> If we really *need* this helper, we should spell out clearly that it is very
> likely the wrong thing to use outside hugetlb code.

I'm also a bit concerned about softleaves here. Kinda implying every pte
has a folio is problematic in general especially if there is nothing
guarding against that being used incorrectly.

pte_page() is more of an low-level arch-helper it seems to me (let's go
look up a PFN from the vmemmap modulo arch stuff around the pte).

So yeah I'm a little iffy about it too! :)

>
> --
> Cheers,
>
> David

--
Cheers, Lorenzo