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

[Jan Kara <jack@suse.cz>]

On Tue 30-07-13 17:26:24, Eric Sandeen wrote:
> 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 /mnt
> 
> 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>
> ---
> 
> The patch is a little hacky, doing all the work in option parsing,
> just to get to a journal devnum like the old option expected, only
> to later re-decode it when we really want to open it.
> 
> I could clean it up so that both journal-update mount options
> find the bdev, rather than ending with an encoded device number,
> which must then be decoded & re-opened, if that seems better.
> 
> But this was expedient enough to get the idea out on the list.
> 
> If we like it, I'll do ext4 as well.
  Yeah, it looks like a good idea. You could just lookup the path via
kern_path() and then take the device numbers from the inode so you won't
have to do the decode-recode dance. That would look like the cleanest option
to me.

								Honza

> 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..cb1dd6e 100644
> --- a/fs/ext3/super.c
> +++ b/fs/ext3/super.c
> @@ -819,6 +819,7 @@ 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 +861,7 @@ 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 +977,9 @@ static int parse_options (char *options, struct super_block *sb,
>  	int option;
>  	kuid_t uid;
>  	kgid_t gid;
> +	struct block_device *journal_bdev;
> +	char *journal_path;
> +
>  #ifdef CONFIG_QUOTA
>  	int qfmt;
>  #endif
> @@ -1129,6 +1134,31 @@ 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;
> +			}
> +			journal_bdev = lookup_bdev(journal_path);
> +			if (IS_ERR(journal_bdev)) {
> +				ext3_msg(sb, KERN_ERR, "error: could not look "
> +					"up journal device: error %d",
> +					(int)PTR_ERR(journal_bdev));
> +				kfree(journal_path);
> +				return 0;
> +			}
> +
> +			*journal_devnum = new_encode_dev(journal_bdev->bd_dev);
> +			bdput(journal_bdev);
> +			kfree(journal_path);
> +			break;
>  		case Opt_noload:
>  			set_opt (sbi->s_mount_opt, NOLOAD);
>  			break;
> 
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR