Re: [PATCH v7 1/3] mm: make persistent huge zero folio read-only

[Xueyuan Chen]

On Mon, Sep 7, 2026 at 10:48 PM David Hildenbrand (Arm)
<david@xxxxxxxxxx> wrote:
>
> On 9/7/26 16:36, Mike Rapoport wrote:
> > On Mon, Sep 07, 2026 at 01:18:42PM +0200, David Hildenbrand (Arm) wrote:
> >> On 9/7/26 08:03, Mike Rapoport wrote:
> >>> Hi Xueyuan,
> >>>
> >>>
> >>> And I believe this is an historical artifact rather than necessity. For
> >>> configurations that allow set_direct_map on arm64, change_memory_common can
> >>> update direct map too, it just need to be implemented there.
> >>>
> >>
> >> Are we sure the other architectures do what we want in their set_memory_ro?
> >
> > It's a zoo right now :(
>
> Yeah, that's why I'm asking :)
>
> > Although most of them do.
> >
> > But we need to sort it out anyway because of increasingly growing demand
> > for modifications of the kernel page tables.
>
> The question really is whether we want to use a single interface for all page
> tables.
>
> I quite like a page-based directmap interface. Other page table areas are not
> necessarily page-based. It also expresses which types of operations we expect on
> the direct map.
>
> One could, of course, use set_memory_ro() internally to implement
> set_direct_map_ro() (and similarly for other variants); but at least
> set_direct_map_*() would have clear, well documented semantics.
>
> >
> > I started something here:
> > https://lore.kernel.org/linux-mm/20260721-generic-set-memory-v0-1-v1-0-2c1fc62306b3@xxxxxxxxxx/
> >
> > Now that x86 CPA is nearly fixed, I can look into the next iteration.
> >
> >> Having a dedicated set_direct_map_ro() with clear (documented ;)) semantics does
> >> not sound too crazy to me.
> >
> > It's not too crazy, no.
> >
> > Still making arm64 more aligned with x86 and other arches that update the
> > direct/linear map in the set_memory_ro() seem a good thing on it's own.
>
> Right, but I'd much rather user set_memory_ro() later to implement
> set_direct_map_ro() [initially on architectures where we made sure that it
> actually does what we expect? later maybe generically]
>

Hi David, Mike,

Thanks for the discussion. I'll keep the page-based set_direct_map_ro()
interface. :)

Thanks,
Xueyuan

> --
> Cheers,
>
> David