Re: Some way of telling which block devices are in use (and how)

[Greg KH <gregkh@linuxfoundation.org>]

On Sun, Apr 29, 2012 at 02:57:11PM -0400, Theodore Ts'o wrote:
> 
> It would be useful if there was a way to be able to determine,
> conclusively, whether a partciular block is in device, and how (i.e.,
> whether some process has the block open, or it is mounted, or it is
> being used as part of a device mapper, or md setup, etc.).
> 
> That way when users complain that trying to open a particular device in
> exclusive mode returns EBUSY, there's an easy way to figure out why this
> might be the case.  The lsof program works for the first method, but it
> doesn't help for the other cases; and using /proc/mounts doesn't help if
> the block device is mounted in some other namespace; you could try to
> look at /proc/*/mounts for every single process, but that's exquisitely
> painful, and only works for block devices used by mounted file systems.
> 
> So the question is, what's the best way of doing this?  Adding a new
> flags field after the block device name in /proc/partitions would be the
> simplest, but that might break some userspace programs that aren't
> expecting it.  There's the holders directory in sysfs, i.e.,
> /sys/dev/block/8:5/holders/*, but that only seems to only record usage
> by device mapper.
> 
> Should we extend the holders directory in the block device directory in
> sysfs?  Add a new directory to record when a file system might be
> mounted?  Add a new /proc file?

Don't we already have a way to do all of this through sysfs today?  How
does gnome/kde handle this when you try to eject a device with an active
mount that is busy?  It pops up a dialog telling you why this can't be
ejected, and which device it is.  I think everything you want is already
there, if not, perhaps dm just needs to add a few more sysfs links to
tie it all together.

And no, no more /proc files for filesystem stuff like this, that way
lies madness.

thanks,

greg k-h