Subject: Re: [PATCH v4 11/15] arm64: mm: Don't abuse memblock NOMAP to check for overlaps

From: Kevin Brodsky <[email-masked]>
Date: Wed, 29 Apr 2026 12:54:20 +0200

On 27/04/2026 17:34, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <[email-masked]>
>
> 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 in order
> to omit text and rodata when creating the linear map.
>
> Instead, map the kernel text and rodata alias first with the desired
> attributes, so that they will not be remapped later with different
> attributes when mapping the memblocks.
>
> Signed-off-by: Ard Biesheuvel <[email-masked]>
> ---
>  arch/arm64/mm/mmu.c | 24 +++++++-------------
>  1 file changed, 8 insertions(+), 16 deletions(-)
>
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 5e2348b15783..1a4b4337d29a 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -1148,12 +1148,15 @@ 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.
>  	 */
> -	memblock_mark_nomap(kernel_start, kernel_end - kernel_start);
> +	__map_memblock(kernel_start, kernel_end, pgprot_tagged(PAGE_KERNEL),

Is it now necessary for that mapping to be tagged? If so, it should
probably be called out in the commit message.

- Kevin

> +		       flags);
>  
>  	/* map all the memory banks */
>  	for_each_mem_range(i, &start, &end) {
> @@ -1165,17 +1168,6 @@ static void __init map_mem(void)
>  		__map_memblock(start, end, pgprot_tagged(PAGE_KERNEL),
>  			       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.
> -	 */
> -	__map_memblock(kernel_start, kernel_end, PAGE_KERNEL, 0);
> -	memblock_clear_nomap(kernel_start, kernel_end - kernel_start);
>  }
>  
>  void mark_rodata_ro(void)