If you're talking about the kernel's filesystem cache, wouldn't that cache the compressed files? As far as I understand it userspace caching is necessary to cache uncompressed blocks, since the decompression is (presumably) done in userspace. I definitely could be wrong though, let me know if you're talking about a different kind of kernel caching.

Actually, I guess if DwarFS is a kernel module and decompresses blocks before they hit the kernel's filesystem cache, then the kernel cache would do it? I'm not sure how to tell from the README if DwarFS is a kernel module or not. So I guess I'm just confused and looking to learn -- what kind of kernel caching did you have in mind?