linux-ext4 - Re: [PATCH 2/3] mke2fs: print extra information about existing ext2/3/4 file systems

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Date:	Mon, 5 May 2014 10:44:23 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Lukáš Czerner <lczerner@...hat.com>
Cc:	Ext4 Developers List <linux-ext4@...r.kernel.org>, kzak@...hat.com,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 2/3] mke2fs: print extra information about existing
 ext2/3/4 file systems

On Mon, May 05, 2014 at 04:38:37PM +0200, Lukáš Czerner wrote:
> > % ./misc/mke2fs -t ext4 /dev/sdc3
> > mke2fs 1.42.9 (4-Feb-2014)
> > /dev/sdc3 contains a ext4 file system
> > 	last mounted on /SOX-backups on Mon May  5 08:59:53 2014
> > Proceed anyway? (y,n) 
> > 
> > ... where this becomes a last-ditch saving through against the
> > accidental wiping of the enterprise's Sarbanes-Oxley records.  :-)
> > 
> 
> Yep, it's really useful. I just was not sure what is this all about since
> there was not description and I was missing context from the other patches.
> 
> But this makes me think that it would be very useful if blkid could
> gather this information for other file system if possible :). This
> might be very useful if we can get some overlap with other file
> system with the information provided in superblock.

Unfortunately, as far as I know, none of the other file systems
currently save the location where the file system was last mounted.
And to be honest, the way we do it in ext4 is a horrible hack (get out
your barf bags!):

static int ext4_file_open(struct inode * inode, struct file * filp)
{
	struct super_block *sb = inode->i_sb;
	struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
	struct vfsmount *mnt = filp->f_path.mnt;
	struct path path;
	char buf[64], *cp;

	if (unlikely(!(sbi->s_mount_flags & EXT4_MF_MNTDIR_SAMPLED) &&
		     !(sb->s_flags & MS_RDONLY))) {
		sbi->s_mount_flags |= EXT4_MF_MNTDIR_SAMPLED;
		/*
		 * Sample where the filesystem has been mounted and
		 * store it in the superblock for sysadmin convenience
		 * when trying to sort through large numbers of block
		 * devices or filesystem images.
		 */
		memset(buf, 0, sizeof(buf));
		path.mnt = mnt;
		path.dentry = mnt->mnt_root;
		cp = d_path(&path, buf, sizeof(buf));
		...

What we would need to do is file a feature request in the other file
systems to save this information, and then add proper support for to
pass this information from the VFS layer into the struct
super_operations's mount function, which would be the proper, sane way
to provide this functionality.

						- Ted




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