From: Carlos Maiolino <cmaiolino@redhat.com>
Subject: Re: [PATCH V2] ext4: allow specifying external journal by pathname
 mount option
Date: Wed, 7 Aug 2013 11:21:20 -0300
Message-ID: <20130807142120.GA10565@andromeda.usersys.redhat.com>
References: <51F98CE4.1050508@redhat.com>
 <51F98DAD.2020909@redhat.com>
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Cc: ext4 development <linux-ext4@vger.kernel.org>
To: Eric Sandeen <sandeen@redhat.com>
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In-Reply-To: <51F98DAD.2020909@redhat.com>
Sender: linux-ext4-owner@vger.kernel.org

This looks good to me, you can add:

Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
On Wed, Jul 31, 2013 at 05:20:29PM -0500, 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 ...
> 
> 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.ext4 -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] EXT4-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: grr, remove debug printk, sorry :/
> 
> p.s. is there any point to MOPT_GTE0?  Doesn't the "=%u" token parsing
> exclude negative numbers?
> 
> I hate to add the MOPT_STRING, but since we try to grab arguments
> outside the case statement, I guess it's required...
> 
> -Eric
> 
> 
> 
> diff --git a/Documentation/filesystems/ext4.txt b/Documentation/filesystems/ext4.txt
> index f7cbf57..b91cfaa 100644
> --- a/Documentation/filesystems/ext4.txt
> +++ b/Documentation/filesystems/ext4.txt
> @@ -144,11 +144,12 @@ journal_async_commit	Commit block can be written to disk without waiting
>  			mount the device. This will enable 'journal_checksum'
>  			internally.
>  
> +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
>  noload			if the filesystem was not unmounted cleanly,
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index bca26f3..2f5ce3a 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -1134,8 +1134,8 @@ enum {
>  	Opt_nouid32, Opt_debug, Opt_removed,
>  	Opt_user_xattr, Opt_nouser_xattr, Opt_acl, Opt_noacl,
>  	Opt_auto_da_alloc, Opt_noauto_da_alloc, Opt_noload,
> -	Opt_commit, Opt_min_batch_time, Opt_max_batch_time,
> -	Opt_journal_dev, Opt_journal_checksum, Opt_journal_async_commit,
> +	Opt_commit, Opt_min_batch_time, Opt_max_batch_time, Opt_journal_dev,
> +	Opt_journal_path, Opt_journal_checksum, Opt_journal_async_commit,
>  	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,
> @@ -1179,6 +1179,7 @@ static const match_table_t tokens = {
>  	{Opt_min_batch_time, "min_batch_time=%u"},
>  	{Opt_max_batch_time, "max_batch_time=%u"},
>  	{Opt_journal_dev, "journal_dev=%u"},
> +	{Opt_journal_path, "journal_path=%s"},
>  	{Opt_journal_checksum, "journal_checksum"},
>  	{Opt_journal_async_commit, "journal_async_commit"},
>  	{Opt_abort, "abort"},
> @@ -1338,6 +1339,7 @@ static int clear_qf_name(struct super_block *sb, int qtype)
>  #define MOPT_NO_EXT2	0x0100
>  #define MOPT_NO_EXT3	0x0200
>  #define MOPT_EXT4_ONLY	(MOPT_NO_EXT2 | MOPT_NO_EXT3)
> +#define MOPT_STRING	0x0400
>  
>  static const struct mount_opts {
>  	int	token;
> @@ -1387,6 +1389,7 @@ static const struct mount_opts {
>  	{Opt_resuid, 0, MOPT_GTE0},
>  	{Opt_resgid, 0, MOPT_GTE0},
>  	{Opt_journal_dev, 0, MOPT_GTE0},
> +	{Opt_journal_path, 0, MOPT_STRING},
>  	{Opt_journal_ioprio, 0, MOPT_GTE0},
>  	{Opt_data_journal, EXT4_MOUNT_JOURNAL_DATA, MOPT_NO_EXT2 | MOPT_DATAJ},
>  	{Opt_data_ordered, EXT4_MOUNT_ORDERED_DATA, MOPT_NO_EXT2 | MOPT_DATAJ},
> @@ -1480,7 +1483,7 @@ static int handle_mount_opt(struct super_block *sb, char *opt, int token,
>  		return -1;
>  	}
>  
> -	if (args->from && match_int(args, &arg))
> +	if (args->from && !(m->flags & MOPT_STRING) && match_int(args, &arg))
>  		return -1;
>  	if (args->from && (m->flags & MOPT_GTE0) && (arg < 0))
>  		return -1;
> @@ -1544,6 +1547,44 @@ static int handle_mount_opt(struct super_block *sb, char *opt, int token,
>  			return -1;
>  		}
>  		*journal_devnum = arg;
> +	} else if (token == Opt_journal_path) {
> +		char *journal_path;
> +		struct inode *journal_inode;
> +		struct path path;
> +		int error;
> +
> +		if (is_remount) {
> +			ext4_msg(sb, KERN_ERR,
> +				 "Cannot specify journal on remount");
> +			return -1;
> +		}
> +		journal_path = match_strdup(&args[0]);
> +		if (!journal_path) {
> +			ext4_msg(sb, KERN_ERR, "error: could not dup "
> +				"journal device string");
> +			return -1;
> +		}
> +
> +		error = kern_path(journal_path, LOOKUP_FOLLOW, &path);
> +		if (error) {
> +			ext4_msg(sb, KERN_ERR, "error: could not find "
> +				"journal device path: error %d", error);
> +			kfree(journal_path);
> +			return -1;
> +		}
> +
> +		journal_inode = path.dentry->d_inode;
> +		if (!S_ISBLK(journal_inode->i_mode)) {
> +			ext4_msg(sb, KERN_ERR, "error: journal path %s "
> +				"is not a block device", journal_path);
> +			path_put(&path);
> +			kfree(journal_path);
> +			return -1;
> +		}
> +
> +		*journal_devnum = new_encode_dev(journal_inode->i_rdev);
> +		path_put(&path);
> +		kfree(journal_path);
>  	} else if (token == Opt_journal_ioprio) {
>  		if (arg > 7) {
>  			ext4_msg(sb, KERN_ERR, "Invalid journal IO priority"
> 
> 
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-- 
Carlos