I'm glad there are about a dozen different file systems that don't suck with VM work and quite relieved BtrFS developers are actively working on improving the case that hurts VM performance.
Having said that, I'd love to know if there are automated tests within the kernel that could verify integrity/correctness/performance of things like filesystem drivers in a simple and automated form. Something like it could prevent surprising developments performance regressions like this and provide better mapping between what you want to do and how you should do it.
The closest thing I'm aware of for doing that currently is the Phoronix Testing Suite; or whatever they're calling it now. It's most certainly not complete but it's the only thing I'm aware of that can do any kind of regression testing like that. In fact it's been worked on recently to make it really easy to do testing with git bisect on the kernel.
Having said that, I'd love to know if there are automated tests within the kernel that could verify integrity/correctness/performance of things like filesystem drivers in a simple and automated form. Something like it could prevent surprising developments performance regressions like this and provide better mapping between what you want to do and how you should do it.