The last time I asked to have the VFS add mountpoint info to the superblock, it got a strong NACK from Al Viro because of the possibility of multiple mountpoints due to bind mounts or namespaces.

I think the current hack is as good as it gets. 

Cheers, Andreas

On May 5, 2014, at 8:44, Theodore Ts'o [off-list ref] wrote:

On Mon, May 05, 2014 at 04:38:37PM +0200, Lukáš Czerner wrote:

quoted

quoted

% ./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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