Re: [PATCH v7 10/15] arm64: mm: Don't abuse memblock NOMAP to check for overlaps

[Kevin Brodsky]

On 29/05/2026 17:02, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@xxxxxxxxxx>
>
> Now that the linear region 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
> initial attributes and granularity, so that the loop iterating over the
> memblocks will not remap it in a manner that prevents it from being
> remapped with updated attributes later.
>
> Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>

Reviewed-by: Kevin Brodsky <kevin.brodsky@xxxxxxx>

> ---
>  arch/arm64/mm/mmu.c | 26 ++++++++------------
>  1 file changed, 10 insertions(+), 16 deletions(-)
>
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 971996e46fd1..dcfca5667e5c 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -1164,12 +1164,17 @@ 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 first
> +	 * so that its write permissions can be removed later without the need
> +	 * to split any block mappings created by the loop below.
> +	 *
> +	 * Write permissions are needed for alternatives patching, and will be
> +	 * removed later by mark_linear_text_alias_ro() above. 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),
> +		       flags);
>  
>  	/* map all the memory banks */
>  	for_each_mem_range(i, &start, &end) {
> @@ -1181,17 +1186,6 @@ 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)