Async readahead poses challenges for regular readahead, because the async nature of it can generate unexpected misses - normal synchronous readahead guarantees that the reading thread completes the readahead before asking for more pages.  But it's possible for async readahead to have been issued and not have completed before the reading thread tries to do another read.  This generates a miss, which causes the readahead code to reset.

At runtime, this is mostly an annoyance with minor performance implications, but it makes testing difficult.

It's not 100% clear to me this is worth the difficulty of solving it, so this ticket for now is to note the problem and sketch possible solutions.