Re: [PATCH] pipe: increase minimum default pipe size to 2 pages

From: Linus Torvalds <torvalds@linux-foundation.org>
To: "Alex Xu (Hello71)" <alex_y_xu@yahoo.ca>
Cc: Greg KH <gregkh@linuxfoundation.org>,
	Christian Brauner <christian@brauner.io>,
	David Howells <dhowells@redhat.com>,
	Linux API <linux-api@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.or>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Nicolas Dichtel <nicolas.dichtel@6wind.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ian Kent <raven@themaw.net>
Subject: Re: [PATCH] pipe: increase minimum default pipe size to 2 pages
Date: Thu, 5 Aug 2021 10:41:23 -0700	[thread overview]
Message-ID: <CAHk-=wgE4XwB2E0=CYB8eqss6WB1+FXgVWZRsUUSWTOeq-kh8w@mail.gmail.com> (raw)
In-Reply-To: <1628172774.4en5vcorw2.none@localhost>

On Thu, Aug 5, 2021 at 7:18 AM Alex Xu (Hello71) <alex_y_xu@yahoo.ca> wrote:
>
> I tested 5.4 and it exhibits the same problem as master using this
> non-racy program. I think the problem goes back to v4.5, the first
> release with 759c01142a ("pipe: limit the per-user amount of pages
> allocated in pipes").

Yeah, our pipe buffer allocation strategy has been fairly consistent,
although the exact locking details etc have certainly changed over
time.

I do think the behavior goes back all the way to that "limit to one
single buffer if you hit the pipe size soft limit" commit, because the
thing that example program tests has been true for the whole time,
afaik: first fill up the first pipe buffer completely, then (a) read
everything but one byte, and then (b) try to write another byte.

Doing (a) will leave the pipe buffer still allocated and in use, and
then (b) will fundamentally want to allocate a new buffer for the new
write.  Which will obviously not then be allowed if we have said "one
pipe buffer only".

So a lot of the code around pipe buffers has changed over the years,
and the exact patterns and timing and wakeups has been completely
rewritten, but that buffer allocation pattern is pretty fundamental
and I don't think that has changed at all.

(A long LONG time ago, we had only one pipe buffer, and it was a
single circular queue, and you never had this kind of "used up one
buffer, need to allocate a new one" issue, so it's not like this goes
back to Linux 0.01, but the pipe buffers go back a _loong_ time).

Allowing two buffers obviously doesn't change the basic pattern at all
- but it means that we will always allow having at least PIPE_BUF
bytes in the pipe. So you can obviously still trigger that "cannot
write any more, will block any future writes", but at that point it's
a clear user bug in thinking that pipes have some infinite buffer
size.

In contrast, expecting pipes to be able to hold 2 bytes at a time is
quite reasonable, with POSIX guaranteeing PIPE_BUF of at least 512
bytes.

I've applied Alex's patch.

                 Linus