ext4: Use ext4 file system driver for ext2/ext3 file system mounts

Add a new config option, CONFIG_EXT4_USE_FOR_EXT23 which if enabled,
will cause ext4 to be used for either ext2 or ext3 file system mounts
when ext2 or ext3 is not enabled in the configuration.

This allows minimalist kernel fanatics to drop to file system drivers
from their compiled kernel with out losing functionality.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 58 insertions, 0 deletions

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 5a2db612950b..30476daf966e 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -3960,6 +3960,58 @@ static int ext4_get_sb(struct file_system_type *fs_type, int flags,
 	return get_sb_bdev(fs_type, flags, dev_name, data, ext4_fill_super,mnt);
 }
 
+#if !defined(CONTIG_EXT2_FS) && defined(CONFIG_EXT4_USE_FOR_EXT23)
+static struct file_system_type ext2_fs_type = {
+	.owner		= THIS_MODULE,
+	.name		= "ext2",
+	.get_sb		= ext4_get_sb,
+	.kill_sb	= kill_block_super,
+	.fs_flags	= FS_REQUIRES_DEV,
+};
+
+static inline void register_as_ext2(void)
+{
+	int err = register_filesystem(&ext2_fs_type);
+	if (err)
+		printk(KERN_WARNING
+		       "EXT4-fs: Unable to register as ext2 (%d)\n", err);
+}
+
+static inline void unregister_as_ext2(void)
+{
+	unregister_filesystem(&ext2_fs_type);
+}
+#else
+static inline void register_as_ext2(void) { }
+static inline void unregister_as_ext2(void) { }
+#endif
+
+#if !defined(CONTIG_EXT3_FS) && defined(CONFIG_EXT4_USE_FOR_EXT23)
+static struct file_system_type ext3_fs_type = {
+	.owner		= THIS_MODULE,
+	.name		= "ext3",
+	.get_sb		= ext4_get_sb,
+	.kill_sb	= kill_block_super,
+	.fs_flags	= FS_REQUIRES_DEV,
+};
+
+static inline void register_as_ext3(void)
+{
+	int err = register_filesystem(&ext3_fs_type);
+	if (err)
+		printk(KERN_WARNING
+		       "EXT4-fs: Unable to register as ext3 (%d)\n", err);
+}
+
+static inline void unregister_as_ext3(void)
+{
+	unregister_filesystem(&ext3_fs_type);
+}
+#else
+static inline void register_as_ext3(void) { }
+static inline void unregister_as_ext3(void) { }
+#endif
+
 static struct file_system_type ext4_fs_type = {
 	.owner		= THIS_MODULE,
 	.name		= "ext4",
@@ -3989,11 +4041,15 @@ static int __init init_ext4_fs(void)
 	err = init_inodecache();
 	if (err)
 		goto out1;
+	register_as_ext2();
+	register_as_ext3();
 	err = register_filesystem(&ext4_fs_type);
 	if (err)
 		goto out;
 	return 0;
 out:
+	unregister_as_ext2();
+	unregister_as_ext3();
 	destroy_inodecache();
 out1:
 	exit_ext4_xattr();
@@ -4009,6 +4065,8 @@ out4:
 
 static void __exit exit_ext4_fs(void)
 {
+	unregister_as_ext2();
+	unregister_as_ext3();
 	unregister_filesystem(&ext4_fs_type);
 	destroy_inodecache();
 	exit_ext4_xattr();