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

[Ryan Roberts <ryan.roberts@arm.com>]

On 26/01/2026 09:26, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@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?

> 
> Signed-off-by: Ard Biesheuvel <ardb@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.

I think we could relax this?

Thanks,
Ryan

>  
>  	/* map all the memory banks */
>  	for_each_mem_range(i, &start, &end) {
> @@ -1167,18 +1172,6 @@ static void __init map_mem(void)
>  			       flags);
>  	}
>  
> -	/*
> -	 * 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() below (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.
> -	 */
> -	__map_memblock(kernel_start, kernel_end, PAGE_KERNEL, NO_CONT_MAPPINGS);
> -	memblock_clear_nomap(kernel_start, kernel_end - kernel_start);
>  	arm64_kfence_map_pool(early_kfence_pool);
>  }
>