Re: [PATCH v2] sparc64: Define p4d_page() instead of stubbing it to NULL

[Nathan Chancellor]

On Thu, Oct 08, 2026 at 12:17:11PM -0700, Rosen Penev wrote:
> A sparc64 allmodconfig build with LLVM fails in vmalloc_to_page():
> 
>   mm/vmalloc.c: error: performing pointer arithmetic on a null pointer
>   has undefined behavior [-Werror,-Wnull-pointer-arithmetic]
> 
> sparc64 defines p4d_page() as NULL, on the assumption that it is only
> used by stubbed out hugetlb gup code. Generic code now uses it in
> vmalloc_to_page(), ptdump and KASAN. In vmalloc_to_page() the NULL is
> used in pointer arithmetic, which clang diagnoses even though the
> branch is unreachable on sparc64.
> 
> Define p4d_page() as a NULL function to silence clang's warning on an
> unreachable branch. sparc64 has no p4d leaf mappings, so there is no
> functional change.
> 
> Assisted-by: LLM
> Signed-off-by: Rosen Penev <rosenp@xxxxxxxxx>
> ---
>  v2: use a NULL function instead.
>  arch/sparc/include/asm/pgtable_64.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/sparc/include/asm/pgtable_64.h b/arch/sparc/include/asm/pgtable_64.h
> index 35a217bf6e5c..54e563eb0efd 100644
> --- a/arch/sparc/include/asm/pgtable_64.h
> +++ b/arch/sparc/include/asm/pgtable_64.h
> @@ -851,7 +851,7 @@ static inline pmd_t *pud_pgtable(pud_t pud)
>  #define p4d_clear(p4dp)			(p4d_val(*(p4dp)) = 0UL)
>  
>  /* only used by the stubbed out hugetlb gup code, should never be called */
> -#define p4d_page(p4d)			NULL
> +#define p4d_page(p4d)			static inline struct page *p4d_page(p4d_t p4d) { return NULL; }

Huh? Why did you keep the macro instead of replacing it with the static
inline function wholesale? This should have just be

-#define p4d_page(p4d)			NULL
+static inline struct page *p4d_page(p4d_t p4d) { return NULL; }

no?

>  #define pud_leaf pud_leaf
>  static inline bool pud_leaf(pud_t pud)
> -- 
> 2.56.0
> 

-- 
Cheers,
Nathan