From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1751092AbdAMUrg (ORCPT <rfc822;w@1wt.eu>);
        Fri, 13 Jan 2017 15:47:36 -0500
Received: from zeniv.linux.org.uk ([195.92.253.2]:56886 "EHLO
        ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1750805AbdAMUrf (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Fri, 13 Jan 2017 15:47:35 -0500
Date: Fri, 13 Jan 2017 20:47:31 +0000
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "Alan J. Wylie" <alan@wylie.me.uk>,
        Thorsten Leemhuis <regressions@leemhuis.info>,
        linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: 4.9.0 regression in pipe-backed iov_iter with systemd-nspawn
Message-ID: <20170113204731.GS1555@ZenIV.linux.org.uk>
References: <CA+55aFybndhA_RPd6gAPmbYYzOL5Le1tCjb2GYVGgFVU8xoD+g@mail.gmail.com>
 <22648.32903.752857.203733@wylie.me.uk>
 <20170113093359.GJ1555@ZenIV.linux.org.uk>
 <22648.41914.351371.678606@wylie.me.uk>
 <20170113102019.GK1555@ZenIV.linux.org.uk>
 <20170113111842.GL1555@ZenIV.linux.org.uk>
 <CA+55aFw0T1wARr8kLvcuHkA4bDE+YJ18wKtPaxc1vFHWPgfQQg@mail.gmail.com>
 <20170113200826.GP1555@ZenIV.linux.org.uk>
 <20170113201121.GQ1555@ZenIV.linux.org.uk>
 <CA+55aFwVgk8QwZKeU2xUGxS0rjRUsSKRYVVA-p+vHnaS5ixoAA@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CA+55aFwVgk8QwZKeU2xUGxS0rjRUsSKRYVVA-p+vHnaS5ixoAA@mail.gmail.com>
User-Agent: Mutt/1.7.1 (2016-10-04)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Fri, Jan 13, 2017 at 12:32:37PM -0800, Linus Torvalds wrote:

> Ugh. I still think your patch is butt-ugly, and the index comparisons
> make me nervous, but..

No arguments here - 6am on 20-odd hours of uptime is _not_ a good time
for writing, especially since the data structure needs better documentation
and probably a couple of inlined helpers.  I'll try to massage the damn
thing into more readable form.

> Let's see if Alan's issue actually goes away with your later patch.

*nod*

I'm fairly sure this is the root cause of what's going on there, but it would
be nice to have an experimental confirmation.