Ext4 data corruption trouble [Updated]

A bit more info: this is observed only if journal_checksum is on (I wasn't using it because I knew it was dangerous, but hadn't noticed that journal_async_commit implied journal_checksum). journal_async_commit combined with nobarrier is even worse: on remount after umounting with those options (and rebooting right after), you don't get a journal abort and readonly remount, you get a remount with no indication of corrupted journal.

(nobarrier on its own, without journal_checksum or anything that implies it, seems to be fine, as long as you have suitable battery-backed hardware of course.)