RISC-V: Acquire mmap lock before invoking walk_page_range

As per walk_page_range documentation, mmap lock should be acquired by the
caller before invoking walk_page_range. mmap_assert_locked gets triggered
without that. The details can be found here.

http://lists.infradead.org/pipermail/linux-riscv/2020-June/010335.html

Fixes: 395a21ff(riscv: add ARCH_HAS_SET_DIRECT_MAP support)
Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Michel Lespinasse <walken@google.com>
Reviewed-by: Zong Li <zong.li@sifive.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/mm/pageattr.c

@@ -151,6 +151,7 @@ int set_memory_nx(unsigned long addr, int numpages)
 
 int set_direct_map_invalid_noflush(struct page *page)
 {
+	int ret;
 	unsigned long start = (unsigned long)page_address(page);
 	unsigned long end = start + PAGE_SIZE;
 	struct pageattr_masks masks = {
@@ -158,11 +159,16 @@ int set_direct_map_invalid_noflush(struct page *page)
 		.clear_mask = __pgprot(_PAGE_PRESENT)
 	};
 
-	return walk_page_range(&init_mm, start, end, &pageattr_ops, &masks);
+	mmap_read_lock(&init_mm);
+	ret = walk_page_range(&init_mm, start, end, &pageattr_ops, &masks);
+	mmap_read_unlock(&init_mm);
+
+	return ret;
 }
 
 int set_direct_map_default_noflush(struct page *page)
 {
+	int ret;
 	unsigned long start = (unsigned long)page_address(page);
 	unsigned long end = start + PAGE_SIZE;
 	struct pageattr_masks masks = {
@@ -170,7 +176,11 @@ int set_direct_map_default_noflush(struct page *page)
 		.clear_mask = __pgprot(0)
 	};
 
-	return walk_page_range(&init_mm, start, end, &pageattr_ops, &masks);
+	mmap_read_lock(&init_mm);
+	ret = walk_page_range(&init_mm, start, end, &pageattr_ops, &masks);
+	mmap_read_unlock(&init_mm);
+
+	return ret;
 }
 
 void __kernel_map_pages(struct page *page, int numpages, int enable)