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

[Jürgen Groß]

On 28.02.26 08:09, 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@xxxxxxxxxx>
> ---
>   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)))
>   
>   #define pte_none(pte)		(!pte_val(pte))
>   #define pte_page(pte)		(pfn_to_page(pte_pfn(pte)))
> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> index 1662c5a8f445..a4dbd81d42bf 100644
> --- a/arch/x86/include/asm/pgtable.h
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -738,6 +738,10 @@ static inline pgprotval_t check_pgprot(pgprot_t pgprot)
>   static inline pte_t pfn_pte(unsigned long page_nr, pgprot_t pgprot)
>   {
>   	phys_addr_t pfn = (phys_addr_t)page_nr << PAGE_SHIFT;
> +
> +	/* Filter out _PAGE_PSE to ensure PTEs never carry the huge page bit */
> +	pgprot = __pgprot(pgprot_val(pgprot) & ~_PAGE_PSE);

Is it really a good idea to silently drop the bit?

Today it can either be used for a large page (which should be a pmd,
of course), or - much worse - you'd strip the _PAGE_PAT bit, which is
at the same position in PTEs.

So basically you are removing the ability to use some cache modes.

NACK!


Juergen

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