The Linux-specific MADV_POPULATE_READ is much more reliable than MADV_WILLNEED if you want to implement read-ahead for a memory-mapped file.

In my opinion, the POSIX advices specified for madvise are useless or even dangerous.

On Linux, for precise control of memory-mapped files one should use only these 4 Linux-specific advices: MADV_COLD, MADV_PAGEOUT, MADV_POPULATE_READ & MADV_POPULATE_WRITE.

These should be used within io_uring, so that they will be executed asynchronously.

These have a well-documented meaning and using them carefully should be sufficient to reach optimum performance with mmap.

Though at that point you can just use FADV_WILLNEED.