That's been implemented; in Linux 6.11 bcachefs will correct errors on read. See
> - Self healing on read IO/checksum error
in https://lore.kernel.org/linux-bcachefs/73rweeabpoypzqwyxa7hl...
Making it possible to scrub from userspace by walking and reading everything (tar -c /mnt/bcachefs >/dev/null).
Repro: supposedly only good copy is copied to ram, ram corrupts bit, crc is recalculated using corrupted but, corrupted copy is written back to disk(s).
Why would it need to recalculate the CRC? The correct CRC (or other hash) for the data is already stored in the metadata trees; it's how it discovered that the data was corrupted in the first place. If it writes back corrupted data, it will be detected as corrupted again the next time.
That's how bcachefs is designed right now.
Page cache is a different story, but doesn't apply to what we're talking about here.
Our RAM should all be ECC and our OSes should all be on self-healing filesystems.