Created attachment 182131 [details] dmesg This is reproduced in qemu-kvm where both host and guest use kernel 4.2.0-0.rc1.git0.1.fc23.x86_64. The guest has two ext4 file systems available on two separate virtio block devices: the boot OS remains ext4, and the 2nd ext4 is what's being converted. Both of these ext4 file systems are freshly created just for this report, and have Fedora 22 installed onto them plus a large Fedora 22 ISO of 1.5GB. 1. btrfs-convert /dev/vdb1 2. mount /dev/vdb1 /mnt 3. btrfs sub del /mnt/ext2_saved wait for about 2 minutes 4. btrfs fi defrag -r /mnt completes 5. btrfs balance start /mnt continues for about 1 minute then segmentation fault The host