Re: [PATCH] powerpc/s64: Clarify that radix lacks DEBUG_PAGEALLOC

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

Le 12/10/2021 à 03:13, Joel Stanley a écrit :
> The page_alloc.c code will call into __kernel_map_pages when
> DEBUG_PAGEALLOC is configured and enabled.
> 
> As the implementation assumes hash, this should crash spectacularly if
> not for a bit of luck in __kernel_map_pages. In this function
> linear_map_hash_count is always zero, the for loop exits without doing
> any damage.
> 
> There are no other platforms that determine if they support
> debug_pagealloc at runtime. Instead of adding code to mm/page_alloc.c to
> do that, this change turns the map/unmap into a noop when in radix
> mode and prints a warning once.
> 
> Signed-off-by: Joel Stanley <joel@jms.id.au>
> ---
> I noticed this when I was looking at adding kfence support a while back.
> I've put that work aside and jpn has since gotten further than me, but I
> think this is a fix worth considering.
> 
>   arch/powerpc/include/asm/book3s/64/hash.h |  2 ++
>   arch/powerpc/mm/book3s64/hash_utils.c     |  2 +-
>   arch/powerpc/mm/book3s64/pgtable.c        | 12 ++++++++++++
>   3 files changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/include/asm/book3s/64/hash.h b/arch/powerpc/include/asm/book3s/64/hash.h
> index d959b0195ad9..674fe0e890dc 100644
> --- a/arch/powerpc/include/asm/book3s/64/hash.h
> +++ b/arch/powerpc/include/asm/book3s/64/hash.h
> @@ -255,6 +255,8 @@ int hash__create_section_mapping(unsigned long start, unsigned long end,
>   				 int nid, pgprot_t prot);
>   int hash__remove_section_mapping(unsigned long start, unsigned long end);
>   
> +void hash__kernel_map_pages(struct page *page, int numpages, int enable);
> +
>   #endif /* !__ASSEMBLY__ */
>   #endif /* __KERNEL__ */
>   #endif /* _ASM_POWERPC_BOOK3S_64_HASH_H */
> diff --git a/arch/powerpc/mm/book3s64/hash_utils.c b/arch/powerpc/mm/book3s64/hash_utils.c
> index c145776d3ae5..cfd45245d009 100644
> --- a/arch/powerpc/mm/book3s64/hash_utils.c
> +++ b/arch/powerpc/mm/book3s64/hash_utils.c
> @@ -1988,7 +1988,7 @@ static void kernel_unmap_linear_page(unsigned long vaddr, unsigned long lmi)
>   				     mmu_kernel_ssize, 0);
>   }
>   
> -void __kernel_map_pages(struct page *page, int numpages, int enable)
> +void hash__kernel_map_pages(struct page *page, int numpages, int enable)
>   {
>   	unsigned long flags, vaddr, lmi;
>   	int i;
> diff --git a/arch/powerpc/mm/book3s64/pgtable.c b/arch/powerpc/mm/book3s64/pgtable.c
> index 9e16c7b1a6c5..0aefc272cd03 100644
> --- a/arch/powerpc/mm/book3s64/pgtable.c
> +++ b/arch/powerpc/mm/book3s64/pgtable.c
> @@ -526,3 +526,15 @@ static int __init pgtable_debugfs_setup(void)
>   	return 0;
>   }
>   arch_initcall(pgtable_debugfs_setup);
> +
> +#ifdef CONFIG_DEBUG_PAGEALLOC
> +void __kernel_map_pages(struct page *page, int numpages, int enable)
> +{
> +	if (radix_enabled()) {
> +		pr_warn_once("DEBUG_PAGEALLOC not supported in radix mode\n");
> +		return;
> +	}
> +
> +	hash__kernel_map_pages(page, numpages, enable);
> +}

I think it would be better to do similar to most other functions (like 
map_kernel_page() for instance):

In arch/powerpc/include/asm/book3s/64/pgtable.h do:

static inline void __kernel_map_pages(struct page *page, int numpages, 
int enable)
{
	if (radix_enabled())
		radix__kernel_map_pages(...);
	else
		hash__kernel_map_pages(...);
}

Then in arch/powerpc/include/asm/book3s/64/radix.h do (or in 
/arch/powerpc/mm/book3s64/radix_pgtable.c in you prefer ?):

static inline void radix__kernel_map_pages(struct page *page, int 
numpages, int enable)
{
	pr_warn_once("DEBUG_PAGEALLOC not supported in radix mode\n");
}

> +#endif
>