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

[Rosen Penev]

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() in terms of p4d_pgtable(), the same way pud_page() is
defined in terms of pud_pgtable(). sparc64 has no p4d leaf mappings, so
there is no functional change.

Assisted-by: LLM
Signed-off-by: Rosen Penev <rosenp@xxxxxxxxx>
---
 arch/sparc/include/asm/pgtable_64.h | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/sparc/include/asm/pgtable_64.h b/arch/sparc/include/asm/pgtable_64.h
index 35a217bf6e5c..c5eff6efe514 100644
--- a/arch/sparc/include/asm/pgtable_64.h
+++ b/arch/sparc/include/asm/pgtable_64.h
@@ -850,8 +850,7 @@ static inline pmd_t *pud_pgtable(pud_t pud)
 #define p4d_present(p4d)		(p4d_val(p4d) != 0U)
 #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)			virt_to_page((void *)p4d_pgtable(p4d))
 
 #define pud_leaf pud_leaf
 static inline bool pud_leaf(pud_t pud)
-- 
2.56.0