Depends a lot on the memory pressure. If you can be fairly certain the data is (or will be) resident in memory, mmap is basically unbeatable. If you can't (because the data is larger than RAM or there's other stuff competing for RAM), mmap can have gnarly system-wide performance implications[1].

[1] Mandatory mmap=poop-emoji link: https://db.cs.cmu.edu/mmap-cidr2022/

Databases vendors usually find that read outperforms mmap. Mmap being fast is a myth.

Why would you use uring to read from an mmap? Couldn't you just memcpy?