Re: [RFC][PATCH] fix short copy handling in copy_mc_pipe_to_iter()

[Al Viro <viro@zeniv.linux.org.uk>]

On Tue, Jun 14, 2022 at 07:36:19AM +0100, David Howells wrote:
> Al Viro <viro@zeniv.linux.org.uk> 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?