fs/ntfs3: Redesign legacy ntfs support

1) Make is_legacy_ntfs static inline.
2) Put legacy file_operations under #if IS_ENABLED(CONFIG_NTFS_FS).

Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Christian Brauner <brauner@kernel.org>
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>

fs/ntfs3/dir.c

@@ -631,10 +631,12 @@ const struct file_operations ntfs_dir_operations = {
 #endif
 };
 
+#if IS_ENABLED(CONFIG_NTFS_FS)
 const struct file_operations ntfs_legacy_dir_operations = {
 	.llseek		= generic_file_llseek,
 	.read		= generic_read_dir,
 	.iterate_shared	= ntfs_readdir,
 	.open		= ntfs_file_open,
 };
+#endif
 // clang-format on

fs/ntfs3/file.c

@@ -1242,6 +1242,7 @@ const struct file_operations ntfs_file_operations = {
 	.release	= ntfs_file_release,
 };
 
+#if IS_ENABLED(CONFIG_NTFS_FS)
 const struct file_operations ntfs_legacy_file_operations = {
 	.llseek		= generic_file_llseek,
 	.read_iter	= ntfs_file_read_iter,
@@ -1249,4 +1250,5 @@ const struct file_operations ntfs_legacy_file_operations = {
 	.open		= ntfs_file_open,
 	.release	= ntfs_file_release,
 };
+#endif
 // clang-format on

fs/ntfs3/inode.c

@@ -441,10 +441,9 @@ static struct inode *ntfs_read_mft(struct inode *inode,
 		 * Usually a hard links to directories are disabled.
 		 */
 		inode->i_op = &ntfs_dir_inode_operations;
-		if (is_legacy_ntfs(inode->i_sb))
-			inode->i_fop = &ntfs_legacy_dir_operations;
-		else
-			inode->i_fop = &ntfs_dir_operations;
+		inode->i_fop = unlikely(is_legacy_ntfs(sb)) ?
+				       &ntfs_legacy_dir_operations :
+				       &ntfs_dir_operations;
 		ni->i_valid = 0;
 	} else if (S_ISLNK(mode)) {
 		ni->std_fa &= ~FILE_ATTRIBUTE_DIRECTORY;
@@ -454,10 +453,9 @@ static struct inode *ntfs_read_mft(struct inode *inode,
 	} else if (S_ISREG(mode)) {
 		ni->std_fa &= ~FILE_ATTRIBUTE_DIRECTORY;
 		inode->i_op = &ntfs_file_inode_operations;
-		if (is_legacy_ntfs(inode->i_sb))
-			inode->i_fop = &ntfs_legacy_file_operations;
-		else
-			inode->i_fop = &ntfs_file_operations;
+		inode->i_fop = unlikely(is_legacy_ntfs(sb)) ?
+				       &ntfs_legacy_file_operations :
+				       &ntfs_file_operations;
 		inode->i_mapping->a_ops = is_compressed(ni) ? &ntfs_aops_cmpr :
 							      &ntfs_aops;
 		if (ino != MFT_REC_MFT)
@@ -1627,10 +1625,9 @@ int ntfs_create_inode(struct mnt_idmap *idmap, struct inode *dir,
 
 	if (S_ISDIR(mode)) {
 		inode->i_op = &ntfs_dir_inode_operations;
-		if (is_legacy_ntfs(inode->i_sb))
-			inode->i_fop = &ntfs_legacy_dir_operations;
-		else
-			inode->i_fop = &ntfs_dir_operations;
+		inode->i_fop = unlikely(is_legacy_ntfs(sb)) ?
+				       &ntfs_legacy_dir_operations :
+				       &ntfs_dir_operations;
 	} else if (S_ISLNK(mode)) {
 		inode->i_op = &ntfs_link_inode_operations;
 		inode->i_fop = NULL;
@@ -1639,10 +1636,9 @@ int ntfs_create_inode(struct mnt_idmap *idmap, struct inode *dir,
 		inode_nohighmem(inode);
 	} else if (S_ISREG(mode)) {
 		inode->i_op = &ntfs_file_inode_operations;
-		if (is_legacy_ntfs(inode->i_sb))
-			inode->i_fop = &ntfs_legacy_file_operations;
-		else
-			inode->i_fop = &ntfs_file_operations;
+		inode->i_fop = unlikely(is_legacy_ntfs(sb)) ?
+				       &ntfs_legacy_file_operations :
+				       &ntfs_file_operations;
 		inode->i_mapping->a_ops = is_compressed(ni) ? &ntfs_aops_cmpr :
 							      &ntfs_aops;
 		init_rwsem(&ni->file.run_lock);

fs/ntfs3/ntfs_fs.h

@@ -1140,6 +1140,13 @@ static inline void le64_sub_cpu(__le64 *var, u64 val)
 	*var = cpu_to_le64(le64_to_cpu(*var) - val);
 }
 
+#if IS_ENABLED(CONFIG_NTFS_FS)
 bool is_legacy_ntfs(struct super_block *sb);
+#else
+static inline bool is_legacy_ntfs(struct super_block *sb)
+{
+	return false;
+}
+#endif
 
 #endif /* _LINUX_NTFS3_NTFS_FS_H */

fs/ntfs3/super.c

@@ -1837,10 +1837,8 @@ bool is_legacy_ntfs(struct super_block *sb)
 #else
 static inline void register_as_ntfs_legacy(void) {}
 static inline void unregister_as_ntfs_legacy(void) {}
-bool is_legacy_ntfs(struct super_block *sb) { return false; }
 #endif
 
-
 // clang-format on
 
 static int __init init_ntfs_fs(void)