George,
You've found a bug. I've got a reproducer.
https://github.com/kdave/btrfs-progs/issues/349
Both the ext4 rollback file, ext2_saved/image, and the converted btrfs
file system are OK. The corruption is bogus, so the bug is that
somehow checksums on a handful of specific blocks (not random) are
computed incorrectly. Btrfs integrity checking sees these blocks as
corrupt, refuses to hand over those blocks when trying to mount the
ext4 file system, and therefore mount fails because the ext4
superblock can't be read.
The rollback will still work. But of course that means you lose the
btrfs file system and everything on it - it goes back to the state it
was in before conversion.
Another idea is a workaround that I've tested that's probably a lot
more convenient. First, you need a 5.11 or newer kernel.
mount -o ro,rescue=all /dev/sda8 /mnt/btrfs
mount -o ro,loop /mnt/btrfs/ext2_saved/image /mnt/oldext4
That's it. You'll be able to inspect /mnt/oldext4 for the files you've
accidentally deleted. Of course you can't copy files to /mnt/btrfs
because it's read-only.
--
Chris Murphy