Re: [PATCH v10 09/12] powerpc: mm: Add common pud_pfn stub for all platforms

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Le 13/03/2024 à 05:21, Rohan McLure a écrit :
> Prior to this commit, pud_pfn was implemented with BUILD_BUG as the inline
> function for 64-bit Book3S systems but is never included, as its
> invocations in generic code are guarded by calls to pud_devmap which return
> zero on such systems. A future patch will provide support for page table
> checks, the generic code for which depends on a pud_pfn stub being
> implemented, even while the patch will not interact with puds directly.
> 
> Remove the 64-bit Book3S stub and define pud_pfn to warn on all
> platforms. pud_pfn may be defined properly on a per-platform basis
> should it grow real usages in future.

Can you please re-explain why that's needed ? I remember we discussed it 
already in the past, but I checked again today and can't see the need:

In mm/page_table_check.c, the call to pud_pfn() is gated by a call to 
pud_user_accessible_page(pud). If I look into arm64 version of 
pud_user_accessible_page(), it depends on pud_leaf(). When pud_leaf() is 
constant 0, pud_user_accessible_page() is always false and the call to 
pud_pfn() should be folded away.

> 
> Signed-off-by: Rohan McLure <rmclure@linux.ibm.com>
> ---
>   arch/powerpc/include/asm/pgtable.h | 14 ++++++++++++++
>   1 file changed, 14 insertions(+)
> 
> diff --git a/arch/powerpc/include/asm/pgtable.h b/arch/powerpc/include/asm/pgtable.h
> index 0c0ffbe7a3b5..13f661831333 100644
> --- a/arch/powerpc/include/asm/pgtable.h
> +++ b/arch/powerpc/include/asm/pgtable.h
> @@ -213,6 +213,20 @@ static inline bool arch_supports_memmap_on_memory(unsigned long vmemmap_size)
>   
>   #endif /* CONFIG_PPC64 */
>   
> +/*
> + * Currently only consumed by page_table_check_pud_{set,clear}. Since clears
> + * and sets to page table entries at any level are done through
> + * page_table_check_pte_{set,clear}, provide stub implementation.
> + */
> +#ifndef pud_pfn
> +#define pud_pfn pud_pfn
> +static inline int pud_pfn(pud_t pud)
> +{
> +	WARN_ONCE(1, "pud: platform does not use pud entries directly");
> +	return 0;
> +}
> +#endif
> +
>   #endif /* __ASSEMBLY__ */
>   
>   #endif /* _ASM_POWERPC_PGTABLE_H */