I know people like to post this rant but in this case Linus simply doesn't understand the problem domain. O_DIRECT is commonly used in contexts where the fundam

I know people like to post this rant but in this case Linus simply doesn't understand the problem domain. O_DIRECT is commonly used in contexts where the fundamental mechanisms of the kernel cache are inappropriate. It can't be fixed with hints. As a database example, there are major classes of optimization that require perfect visibility into the state of the entire page cache with virtually no overhead and strict control over every change of state that occurs. O_DIRECT allows you to achieve this. The optimizations are predicated on the impossibility of an external process modifying state. It requires perfect control of the schedule which is invalidated if the kernel borrows part of the page cache. Whether or not the kernel asks nicely doesn't matter, it breaks a design invariant. The Linus rant is from a long time ago. Given the existence of things like io_uring which explicitly enables this type of behavior almost to the point of encouraging it, Linus may understand the use cases better now.