From: Jan Kara Subject: Re: [PATCH] ext3: allow specifying external journal by pathname mount option Date: Wed, 31 Jul 2013 16:05:45 +0200 Message-ID: <20130731140545.GA22930@quack.suse.cz> References: <51F83D90.5080708@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: ext4 development , Jan Kara , Leonardo Menezes Vaz To: Eric Sandeen Return-path: Received: from cantor2.suse.de ([195.135.220.15]:38343 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752816Ab3GaOFr (ORCPT ); Wed, 31 Jul 2013 10:05:47 -0400 Content-Disposition: inline In-Reply-To: <51F83D90.5080708@redhat.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: 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 > --- > > 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 SUSE Labs, CR