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

[Rosen Penev]

On Fri, Oct 9, 2026 at 12:13 PM Nathan Chancellor <nathan@xxxxxxxxxx> wrote:
>
> 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?
Was not thinking. I sent a v3 and actually tested compilation this time.
>
> >  #define pud_leaf pud_leaf
> >  static inline bool pud_leaf(pud_t pud)
> > --
> > 2.56.0
> >
>
> --
> Cheers,
> Nathan