On Tue, Jun 14, 2022 at 07:36:19AM +0100, David Howells wrote: > Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote: > > > What's wrong with > > p_occupancy = pipe_occupancy(head, tail); > > if (p_occupancy >= pipe->max_usage) > > return 0; > > else > > return pipe->max_usage - p_occupancy; > > Because "pipe->max_usage - p_occupancy" can be negative. Sure can. And in that case you return 0; no problem wiht that. It's what happens when occupancy is below max_usage that is weird. > post_one_notification() is limited by pipe->ring_size, not pipe->max_usage. > > The idea is to allow some slack in a watch pipe for the watch_queue code to > use that userspace can't. Sure. And if this function is supposed to report how many times would userspace be able to grab a slot, it's returning the wrong value. Look: 32-slot ring. max_usage is 16. 14 slots are already occupied. Userland (sure as hell, anything in iov_iter.c) will be able to occupy two more before it runs into the pipe_full(). And your function returns min(32 - 14, 16), i.e. 16. What am I missing here?
