See https://lwn.net/Articles/922405/ https://lwn.net/Articles/922405/ for a description of what it does and https://lwn.net/Articles/972710/ https://lwn.net/Art

See https://lwn.net/Articles/922405/ https://lwn.net/Articles/922405/ for a description of what it does and https://lwn.net/Articles/972710/ https://lwn.net/Articles/972710/ for the controversy it caused that is the reason why it took so long to land in mainline.