The important distinction is where the critical information on where the device offset is encoded from. Before it was specified by whether a partition or parent disk device was used; now it is always the actual disk and a partition number. It's not a bad change, but IMO it was made recklessly.

From this patchset, there's now at least four different fixes: tags referring to it in Linus's tree. The patchset itself was broken and intermediate states didn't compile, making bisection annoying. And there's many other callers of this function that are potentially getting bad results, along with other cousins like bio_clone_bioset that are still unfixed.

This was not a fun experience for a new maintainer. It turns out if you had created a fresh bcache on 4.14, like most of my tests do-- you're probably fine. And not all environments will experience problems. It went in a couple of weeks before I became maintainer, and isn't in the bcache code itself (and wasn't reviewed by anyone who worked on bcache, and was never sent to the linux-bcache mailing list.) I've learned some new things to add to my test suite, and I guess i need to read all the block diff to make sure people don't break things out from under me.