With the current code, your data becomes unmountable in that case. There is a workaround for getting the data accessable: leave a bit of the underlying blockdevice unused, so when creating VDO ontop give it not the full blockdevice. Then, in case the filesystem ontop completely fills up, the last part of the blockdevice can be used to grow the VDO device, and the filesystem can be mounted.

Should be tried out before relying on it. The current behaviour seems to be considered a bug, https://bugzilla.redhat.com/show_bug.cgi?id=1519377 has details.