[PATCH v2 08/10] arm64: mm: Don't abuse memblock NOMAP to check for overlaps

[Ard Biesheuvel]

On Tue, 27 Jan 2026 at 11:21, Ryan Roberts <ryan.roberts at arm.com> wrote:
>
> On 26/01/2026 09:26, Ard Biesheuvel wrote:
> > From: Ard Biesheuvel <ardb at kernel.org>
> >
> > Now that the DRAM mapping routines respect existing table mappings and
> > contiguous block and page mappings, it is no longer needed to fiddle
> > with the memblock tables to set and clear the NOMAP attribute. Instead,
> > map the kernel text and rodata alias first, avoiding contiguous
> > mappings, so that they will not be added later when mapping the
> > memblocks.
>
> Should we do something similar for kfence? Currently we have
> arm64_kfence_alloc_pool() which marks some memory NOMAP then
> arm64_kfence_map_pool() which PTE-maps it and clears NOMAP. Presumably we could
> rationalize into a single function that does it all, prior to mapping the bulk
> of the linear map?
>

Yeah, good point - I did not spot that but I will address it in the
next revision.

> >
> > Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> > ---
> >  arch/arm64/mm/mmu.c | 27 ++++++++------------
> >  1 file changed, 10 insertions(+), 17 deletions(-)
> >
> > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> > index 80587cd47ce7..18415d4743bf 100644
> > --- a/arch/arm64/mm/mmu.c
> > +++ b/arch/arm64/mm/mmu.c
> > @@ -1149,12 +1149,17 @@ static void __init map_mem(void)
> >               flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
> >
> >       /*
> > -      * Take care not to create a writable alias for the
> > -      * read-only text and rodata sections of the kernel image.
> > -      * So temporarily mark them as NOMAP to skip mappings in
> > -      * the following for-loop
> > +      * Map the linear alias of the [_text, __init_begin) interval
> > +      * as non-executable now, and remove the write permission in
> > +      * mark_linear_text_alias_ro() above (which will be called after
> > +      * alternative patching has completed). This makes the contents
> > +      * of the region accessible to subsystems such as hibernate,
> > +      * but protects it from inadvertent modification or execution.
> > +      * Note that contiguous mappings cannot be remapped in this way,
> > +      * so we should avoid them here.
> >        */
> > -     memblock_mark_nomap(kernel_start, kernel_end - kernel_start);
> > +     __map_memblock(kernel_start, kernel_end, PAGE_KERNEL,
> > +                    flags | NO_CONT_MAPPINGS);
>
> So the reason to disallow cont mappings is because we need to modify the
> permissions later? It _is_ safe to change permissions on a live contiguous
> mapping in this way. That was clarified in the architecture a couple of years
> back and we rely on it for contpte_wrprotect_ptes(); see comment there.
>

OK, good to know - I was hoping to get your take on this ...

> I think we could relax this?
>

OK, I suppose that means that we can drop the NO_CONT_MAPPINGS here,
but we still need to map the kernel text/rodata alias initially to
ensure that no block mappings are created that would need to broken
down, right?