Accepting discard to free space from disk images

["Devon B." <devon.b@virtualcomplete.com>]

Looking to use btrfs with disk images that contain ext4, xfs, and other
possible filesystems.  One of my main concerns is reclaiming disk space
when files are deleted on the disk image's filesystem.  Using fstrim on
the mount point of the disk image or mounting the disk image (loop) with
discard doesn't appear to pass the freed blocks to btrfs.

The image file on the btrfs filesystem just keeps growing in size.  When
hosting images on ext4, using fstrim or discard frees up the space on
the host filesystem (ext4).

I see discard is pretty well supported for btrfs to the underlying block
devices but is there any way to properly free space on the btrfs host
filesystem from overlying filesystems while they are online?

Thanks.