I exclusively use EXT4, so this is welcome improvements. It's really rock solid and performance is good too. Not sure how relevant these improvements other than measuring them in specific cases will be for my daily usage, but it can't hurt I guess. Article also talks about:

avoiding possible soft lockups or RCU stall conditions, and a variety of bug fixes

Which is impressive, because its pretty darn good already. I wonder if anyone ecountered a bug in the filesystem on a live system in recent years or if this is just some cleanups they found when inspecting the code, without affecting anyone in real world?

But at what cost?

Been using zfs and btrfs for so long I forgot ext4 was a thing. Oh my.