Re: [RFC PATCH 10/34] x86: mm: carve out the generic compile-time folded pgtable case in effective_prot()

[Yeoreum Yun <yeoreum.yun@arm.com>]

On Tue, Jul 14, 2026 at 06:31:22PM +0200, David Hildenbrand (Arm) wrote:
> 
> >> This makes a *ton* of sense in effective_prot() especially. Its entire
> >> job is mirroring the hardware's job of inheriting permissions from
> >> higher levels of the page tables and enforcing them on lower level leaf
> >> entries. If a higher level is folded, there's nothing to inherit.
> >>
> >> I also think it's worth taking a brief pause on the coding to think
> >> about what kind of design would actually be nice here. If the design
> >> really is that the pgd is folded, the 'struct mm_walk_ops' code would
> >> ideally not even call ->pgd_entry(). It would just (for example) *start*
> >> at ->pud_entry() for a 3-level hardware page table.
> >>
> >> If there are no pgds, why bother calling ->pgd_entry()? It couldn't be
> >> done transparently to mm_walk users of course but it could be done
> >> incrementally where users move one at a time over to a new scheme.
> > 
> > Agree. Calling a pXd_entry() about folded one seems meaningless.
> > But seems enough to check mm_pXd_folded() before calling pXd_entry()
> > in each walk_pXd_range().
> 
> Needs some double-checking that callers can handle it (like effective_prot()
> would currently not), but certainly something to look into!

Most of mm_walk_ops *implicitly* handles the folded pgtable case
(e.x) pXd_none() / pXd_present() and other can handle explicitly in
its own handler.

But, IMHO, It would be better to skip calling pXd_entry() in the
walk_pXd_range() by checking the folded and this is clearer conceptually.

For this, it  requires the rewrite the walk_mm() where use only *p4d_entry()*
to gather the leaf/non-leaf information for MGLRU.

-- 
Sincerely,
Yeoreum Yun