The policy code in the kernel that controls file caching and readahead behavior is necessarily one-size-fits-all, and the knobs available to applications to tune it, like madvise() and posix_fadvise(), are fairly blunt hammers. Furthermore, it has been shown that the overhead from user<->kernel domain crossings makes syscall-driven fine-grained policy control ineffective. (Though, that was shown in the past when processors were much slower relative to disks and it may not be true any more.)

Is it possible to use BPF, or create a different BPF-like tool (that is, a small code generator with very simple and very clear safety properties) to allow safe in-kernel fine-grained policy control?

Caution: this is a research project.