powerpc/s64: Clarify that radix lacks DEBUG_PAGEALLOC

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>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[mpe: Reformat if per Christophe's suggestion]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211013213438.675095-1-joel@jms.id.au

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 */

arch/powerpc/include/asm/book3s/64/pgtable.h

@@ -1101,6 +1101,16 @@ static inline void vmemmap_remove_mapping(unsigned long start,
 }
 #endif
 
+#ifdef CONFIG_DEBUG_PAGEALLOC
+static inline void __kernel_map_pages(struct page *page, int numpages, int enable)
+{
+	if (radix_enabled())
+		radix__kernel_map_pages(page, numpages, enable);
+	else
+		hash__kernel_map_pages(page, numpages, enable);
+}
+#endif
+
 static inline pte_t pmd_pte(pmd_t pmd)
 {
 	return __pte_raw(pmd_raw(pmd));

arch/powerpc/include/asm/book3s/64/radix.h

@@ -316,5 +316,8 @@ int radix__create_section_mapping(unsigned long start, unsigned long end,
 				  int nid, pgprot_t prot);
 int radix__remove_section_mapping(unsigned long start, unsigned long end);
 #endif /* CONFIG_MEMORY_HOTPLUG */
+
+void radix__kernel_map_pages(struct page *page, int numpages, int enable);
+
 #endif /* __ASSEMBLY__ */
 #endif

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;

arch/powerpc/mm/book3s64/radix_pgtable.c

@@ -920,6 +920,13 @@ void __meminit radix__vmemmap_remove_mapping(unsigned long start, unsigned long
 #endif
 #endif
 
+#ifdef CONFIG_DEBUG_PAGEALLOC
+void radix__kernel_map_pages(struct page *page, int numpages, int enable)
+{
+	pr_warn_once("DEBUG_PAGEALLOC not supported in radix mode\n");
+}
+#endif
+
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 
 unsigned long radix__pmd_hugepage_update(struct mm_struct *mm, unsigned long addr,