[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <Yqh7B+tVDutCwuG1@ZenIV>
Date: Tue, 14 Jun 2022 13:11:51 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: David Howells <dhowells@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
Dan Williams <dan.j.williams@...el.com>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
linux-fsdevel <linux-fsdevel@...r.kernel.org>,
nvdimm@...ts.linux.dev
Subject: Re: [RFC][PATCH] fix short copy handling in copy_mc_pipe_to_iter()
On Tue, Jun 14, 2022 at 07:36:19AM +0100, David Howells wrote:
> Al Viro <viro@...iv.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?
Powered by blists - more mailing lists