[PATCH V2] ext3: allow specifying external journal by pathname mount option

[Eric Sandeen <sandeen@redhat.com>]

It's always been a hassle that if an external journal's
device number changes, the filesystem won't mount.
And since boot-time enumeration can change, device number
changes aren't unusual.

The current mechanism to update the journal location is by
passing in a mount option w/ a new devnum, but that's a hassle;
it's a manual approach, fixing things after the fact.

Adding a mount option, "-o journal_path=/dev/$DEVICE" would
help, since then we can do i.e.

# mount -o journal_path=/dev/disk/by-label/$JOURNAL_LABEL ...

and it'll mount even if the devnum has changed, as shown here:

# losetup /dev/loop0 journalfile
# mke2fs -L mylabel-journal -O journal_dev /dev/loop0 
# mkfs.ext3 -L mylabel -J device=/dev/loop0 /dev/sdb1

Change the journal device number:

# losetup -d /dev/loop0
# losetup /dev/loop1 journalfile 

And today it will fail:

# mount /dev/sdb1 /mnt/test
mount: wrong fs type, bad option, bad superblock on /dev/sdb1,
       missing codepage or helper program, or other error
       In some cases useful info is found in syslog - try
       dmesg | tail  or so

# dmesg | tail -n 1
[17343.240702] EXT3-fs (sdb1): error: couldn't read superblock of external journal

But with this new mount option, we can specify the new path:

# mount -o journal_path=/dev/loop1 /dev/sdb1 /mnt/test
#

(which does update the encoded device number, incidentally):

# umount /dev/sdb1
# dumpe2fs -h /dev/sdb1 | grep "Journal device"
dumpe2fs 1.41.12 (17-May-2010)
Journal device:	          0x0701

But best of all we can just always mount by journal-path, and
it'll always work:

# mount -o journal_path=/dev/disk/by-label/mylabel-journal /dev/sdb1 /mnt/test
#

So the journal_path option can be specified in fstab, and as long as
the disk is available somewhere, and findable by label (or by UUID),
we can mount.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
---

V2: get device out of bdev's inode, rather than grabbing bdev & releasing it

diff --git a/Documentation/filesystems/ext3.txt b/Documentation/filesystems/ext3.txt
index 293855e..7ed0d17 100644
--- a/Documentation/filesystems/ext3.txt
+++ b/Documentation/filesystems/ext3.txt
@@ -26,11 +26,12 @@ journal=inum		When a journal already exists, this option is ignored.
 			Otherwise, it specifies the number of the inode which
 			will represent the ext3 file system's journal file.
 
+journal_path=path
 journal_dev=devnum	When the external journal device's major/minor numbers
-			have changed, this option allows the user to specify
+			have changed, these options allow the user to specify
 			the new journal location.  The journal device is
-			identified through its new major/minor numbers encoded
-			in devnum.
+			identified through either its new major/minor numbers
+			encoded in devnum, or via a path to the device.
 
 norecovery		Don't load the journal on mounting. Note that this forces
 noload			mount of inconsistent filesystem, which can lead to
diff --git a/fs/ext3/super.c b/fs/ext3/super.c
index c47f147..c50c761 100644
--- a/fs/ext3/super.c
+++ b/fs/ext3/super.c
@@ -27,6 +27,7 @@
 #include <linux/seq_file.h>
 #include <linux/log2.h>
 #include <linux/cleancache.h>
+#include <linux/namei.h>
 
 #include <asm/uaccess.h>
 
@@ -819,6 +820,7 @@ enum {
 	Opt_user_xattr, Opt_nouser_xattr, Opt_acl, Opt_noacl,
 	Opt_reservation, Opt_noreservation, Opt_noload, Opt_nobh, Opt_bh,
 	Opt_commit, Opt_journal_update, Opt_journal_inum, Opt_journal_dev,
+	Opt_journal_path,
 	Opt_abort, Opt_data_journal, Opt_data_ordered, Opt_data_writeback,
 	Opt_data_err_abort, Opt_data_err_ignore,
 	Opt_usrjquota, Opt_grpjquota, Opt_offusrjquota, Opt_offgrpjquota,
@@ -860,6 +862,7 @@ static const match_table_t tokens = {
 	{Opt_journal_update, "journal=update"},
 	{Opt_journal_inum, "journal=%u"},
 	{Opt_journal_dev, "journal_dev=%u"},
+	{Opt_journal_path, "journal_path=%s"},
 	{Opt_abort, "abort"},
 	{Opt_data_journal, "data=journal"},
 	{Opt_data_ordered, "data=ordered"},
@@ -975,6 +978,11 @@ static int parse_options (char *options, struct super_block *sb,
 	int option;
 	kuid_t uid;
 	kgid_t gid;
+	char *journal_path;
+	struct inode *journal_inode;
+	struct path path;
+	int error;
+
 #ifdef CONFIG_QUOTA
 	int qfmt;
 #endif
@@ -1129,6 +1137,41 @@ static int parse_options (char *options, struct super_block *sb,
 				return 0;
 			*journal_devnum = option;
 			break;
+		case Opt_journal_path:
+			if (is_remount) {
+				ext3_msg(sb, KERN_ERR, "error: cannot specify "
+				       "journal on remount");
+				return 0;
+			}
+
+			journal_path = match_strdup(&args[0]);
+			if (!journal_path) {
+				ext3_msg(sb, KERN_ERR, "error: could not dup "
+					"journal device string");
+				return 0;
+			}
+
+			error = kern_path(journal_path, LOOKUP_FOLLOW, &path);
+			if (error) {
+				ext3_msg(sb, KERN_ERR, "error: could not find "
+					"journal device path: error %d", error);
+				kfree(journal_path);
+				return 0;
+			}
+
+			journal_inode = path.dentry->d_inode;
+			if (!S_ISBLK(journal_inode->i_mode)) {
+				ext3_msg(sb, KERN_ERR, "error: journal path %s "
+					"is not a block device", journal_path);
+				path_put(&path);
+				kfree(journal_path);
+				return 0;
+			}
+
+			*journal_devnum = new_encode_dev(journal_inode->i_rdev);
+			path_put(&path);
+			kfree(journal_path);
+			break;
 		case Opt_noload:
 			set_opt (sbi->s_mount_opt, NOLOAD);
 			break;