Re: [PATCH v2 3/3] arm64: mm: Tidy up force_pte_mapping()

[Yang Shi <yang@os.amperecomputing.com>]

On 11/6/25 8:09 AM, Ryan Roberts wrote:
> Tidy up the implementation of force_pte_mapping() to make it easier to
> read and introduce the split_leaf_mapping_possible() helper to reduce
> code duplication in split_kernel_leaf_mapping() and
> arch_kfence_init_pool().
>
> Suggested-by: David Hildenbrand (Red Hat) <david@kernel.org>
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>

Reviewed-by: Yang Shi <yang@os.amperecomputing.com>

Thanks,
Yang

> ---
>   arch/arm64/mm/mmu.c | 43 +++++++++++++++++++++++--------------------
>   1 file changed, 23 insertions(+), 20 deletions(-)
>
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 652bb8c14035..2ba01dc8ef82 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -710,12 +710,26 @@ static int split_kernel_leaf_mapping_locked(unsigned long addr)
>   
>   static inline bool force_pte_mapping(void)
>   {
> -	bool bbml2 = system_capabilities_finalized() ?
> +	const bool bbml2 = system_capabilities_finalized() ?
>   		system_supports_bbml2_noabort() : cpu_supports_bbml2_noabort();
>   
> -	return (!bbml2 && (rodata_full || arm64_kfence_can_set_direct_map() ||
> -			   is_realm_world())) ||
> -		debug_pagealloc_enabled();
> +	if (debug_pagealloc_enabled())
> +		return true;
> +	if (bbml2)
> +		return false;
> +	return rodata_full || arm64_kfence_can_set_direct_map() || is_realm_world();
> +}
> +
> +static inline bool split_leaf_mapping_possible(void)
> +{
> +	/*
> +	 * !BBML2_NOABORT systems should never run into scenarios where we would
> +	 * have to split. So exit early and let calling code detect it and raise
> +	 * a warning.
> +	 */
> +	if (!system_supports_bbml2_noabort())
> +		return false;
> +	return !force_pte_mapping();
>   }
>   
>   static DEFINE_MUTEX(pgtable_split_lock);
> @@ -725,22 +739,11 @@ int split_kernel_leaf_mapping(unsigned long start, unsigned long end)
>   	int ret;
>   
>   	/*
> -	 * !BBML2_NOABORT systems should not be trying to change permissions on
> -	 * anything that is not pte-mapped in the first place. Just return early
> -	 * and let the permission change code raise a warning if not already
> -	 * pte-mapped.
> -	 */
> -	if (!system_supports_bbml2_noabort())
> -		return 0;
> -
> -	/*
> -	 * If the region is within a pte-mapped area, there is no need to try to
> -	 * split. Additionally, CONFIG_DEBUG_PAGEALLOC and CONFIG_KFENCE may
> -	 * change permissions from atomic context so for those cases (which are
> -	 * always pte-mapped), we must not go any further because taking the
> -	 * mutex below may sleep.
> +	 * Exit early if the region is within a pte-mapped area or if we can't
> +	 * split. For the latter case, the permission change code will raise a
> +	 * warning if not already pte-mapped.
>   	 */
> -	if (force_pte_mapping() || is_kfence_address((void *)start))
> +	if (!split_leaf_mapping_possible() || is_kfence_address((void *)start))
>   		return 0;
>   
>   	/*
> @@ -1039,7 +1042,7 @@ bool arch_kfence_init_pool(void)
>   	int ret;
>   
>   	/* Exit early if we know the linear map is already pte-mapped. */
> -	if (!system_supports_bbml2_noabort() || force_pte_mapping())
> +	if (!split_leaf_mapping_possible())
>   		return true;
>   
>   	/* Kfence pool is already pte-mapped for the early init case. */