[danarmak]

That the process/thread enters kernel mode and then is suspended waiting for IO or for some other event. As long as the thread is running your code (or, is scheduleable) it's not blocked. And then the async implementation can ensure your code cooperatively gives up the CPU for other code.

[magicalhippo]

If your memory is paged out and you then access it, using your definition, it would block.

So, in the context of async code, there's no difference from the application perspective between reading mmap'ed data and reading "regular" data (ie memory from the regular paged pool), as both could incur blocking IO.

If you're lucky and the mmap'ed data is in the system cache, then reading that data will not block and is fast. If you're unlucky and your process has been swapped out, then doing a regular memory read will block and is slow.