Re: [PATCH RFC v3 2/4] mm/pgtable: Make pfn_pte() filter out huge page attributes

[Will Deacon <will@kernel.org>]

On Sat, Feb 28, 2026 at 03:09:04PM +0800, Yin Tirui wrote:
> A fundamental principle of page table type safety is that `pte_t` represents
> the lowest level page table entry and should never carry huge page attributes.
> 
> Currently, passing a pgprot with huge page bits (e.g., extracted via
> pmd_pgprot()) into pfn_pte() creates a malformed PTE that retains the huge
> attribute, leading to the necessity of the ugly `pte_clrhuge()` anti-pattern.
> 
> Enforce type safety by making `pfn_pte()` inherently filter out huge page
> attributes:
> - On x86: Strip the `_PAGE_PSE` bit.
> - On ARM64: Mask out the block descriptor bits in `PTE_TYPE_MASK` and
>   enforce the `PTE_TYPE_PAGE` format.
> - On RISC-V: No changes required, as RISC-V leaf PMDs and PTEs share the
>   exact same hardware format and do not use a distinct huge bit.
> 
> Signed-off-by: Yin Tirui <yintirui@huawei.com>
> ---
>  arch/arm64/include/asm/pgtable.h | 4 +++-
>  arch/x86/include/asm/pgtable.h   | 4 ++++
>  2 files changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index b3e58735c49b..f2a7a40106d2 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -141,7 +141,9 @@ static inline pteval_t __phys_to_pte_val(phys_addr_t phys)
>  
>  #define pte_pfn(pte)		(__pte_to_phys(pte) >> PAGE_SHIFT)
>  #define pfn_pte(pfn,prot)	\
> -	__pte(__phys_to_pte_val((phys_addr_t)(pfn) << PAGE_SHIFT) | pgprot_val(prot))
> +	__pte(__phys_to_pte_val((phys_addr_t)(pfn) << PAGE_SHIFT) | \
> +		((pgprot_val(prot) & ~(PTE_TYPE_MASK & ~PTE_VALID)) | \
> +		(PTE_TYPE_PAGE & ~PTE_VALID)))

Why are you touching arch/arm64? We don't implement pte_clrhuge() afaict.
What does this actually fix?

Will