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

Theodore Ts'o
2014-05-05T14:44:23Z

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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