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

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

On Mon, Jun 13, 2022 at 11:28:34PM +0100, Al Viro wrote:

> Dave, could you explain what's going on there?  Note that pipe_write()
> does *not* use that thing at all; it's only splice (i.e. ITER_PIPE
> stuff) that is using it.
> 
> 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;
> 
> which would match the way you are using ->max_usage in pipe_write()
> et.al.  Including the use in copy_page_to_iter_pipe(), BTW...

The more I'm looking at that thing, the more it smells like a bug;
it had the same 3 callers since the time it had been introduced.

1) pipe_get_pages().  We are about to try and allocate up to that
many pipe buffers.  Allocation (done in push_pipe()) is done only
if we have !pipe_full(pipe->head, pipe->tail, pipe->max_usage).

It simply won't give you more than max_usage - occupancy.
Your function returns min(ring_size - occupancy, max_usage), which
is always greater than or equal to that (ring_size >= max_usage).

2) pipe_get_pages_alloc().  Same story, same push_pipe() being
called, same "we'll never get that much - it'll hit the limit
first".

3) iov_iter_npages() in case of ITER_PIPE.  Again, the value
is bogus - it should not be greater than the amount of pages
we would be able to write there.

AFAICS, 6718b6f855a0 "pipe: Allow pipes to have kernel-reserved slots"
broke it for cases when ring_size != max_usage...