[dm-devel] dm-thin discard issue

[thornber at redhat.com]

Hi Jim,

On Wed, Mar 06, 2013 at 02:26:22PM +0000, thornber at redhat.com wrote:
> On Wed, Mar 06, 2013 at 08:12:39AM -0500, Jim Minter wrote:
> > Hello,
> > 
> > I think I've uncovered a problem when issuing the BLKDISCARD ioctl to a thin volume.  If I create a thin volume, fill it with data, snapshot it, then call BLKDISCARD on the thin volume, it looks like the kernel doesn't take into account the fact that the underlying blocks are shared with the snapshot, and just goes ahead and discards them.  This appears to then leave the metadata in an inconsistent state.
> > 
> > Here's a reproducer (works on rawhide as of today, 3.9.0-0.rc1.git0.1.fc19.x86_64):
> > (assumes volumes 253:2 for metadata and 253:3 for pool; uses blkdiscard from upstream util-linux to issue the BLKDISCARD ioctl)
> 
> Alarming, to say the least.  Give me a couple of hours to look at this ...

I managed to reproduce with this test:

  def test_discard_origin_does_not_effect_snap
    with_standard_pool(@size) do |pool|
      with_new_thin(pool, @volume_size, 0) do |thin|
        wipe_device(thin)
        assert_used_blocks(pool, @blocks_per_dev)

        with_new_snap(pool, @volume_size, 1, 0, thin) do |snap|
          assert_used_blocks(pool, @blocks_per_dev)
        end

        thin.discard(0, @volume_size)
        assert_used_blocks(pool, @blocks_per_dev)
      end
      assert_used_blocks(pool, @blocks_per_dev)
    end
  end

This commit fixes the issue:

     https://github.com/jthornber/linux-2.6/commit/a42dfef751cb666d3274346c07dff655cb40cc5a


I'm really sorry about this, we should have covered this better in the
tests.

- Joe