riscv: add ARCH_SUPPORTS_DEBUG_PAGEALLOC support

ARCH_SUPPORTS_DEBUG_PAGEALLOC provides a hook to map and unmap
pages for debugging purposes. Implement the __kernel_map_pages
functions to fill the poison pattern.
Signed-off-by: Zong Li <zong.li@sifive.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/Kconfig

@@ -131,6 +131,9 @@ config ARCH_SELECT_MEMORY_MODEL
 config ARCH_WANT_GENERAL_HUGETLB
 	def_bool y
 
+config ARCH_SUPPORTS_DEBUG_PAGEALLOC
+	def_bool y
+
 config SYS_SUPPORTS_HUGETLBFS
 	def_bool y
 

arch/riscv/mm/pageattr.c

@@ -172,3 +172,16 @@ int set_direct_map_default_noflush(struct page *page)
 
 	return walk_page_range(&init_mm, start, end, &pageattr_ops, &masks);
 }
+
+void __kernel_map_pages(struct page *page, int numpages, int enable)
+{
+	if (!debug_pagealloc_enabled())
+		return;
+
+	if (enable)
+		__set_memory((unsigned long)page_address(page), numpages,
+			     __pgprot(_PAGE_PRESENT), __pgprot(0));
+	else
+		__set_memory((unsigned long)page_address(page), numpages,
+			     __pgprot(0), __pgprot(_PAGE_PRESENT));
+}