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 S1752715AbbHENNY (ORCPT <rfc822;w@1wt.eu>);
	Wed, 5 Aug 2015 09:13:24 -0400
Received: from mail-vk0-f46.google.com ([209.85.213.46]:36072 "EHLO
	mail-vk0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1751390AbbHENNX (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Wed, 5 Aug 2015 09:13:23 -0400
MIME-Version: 1.0
In-Reply-To: <20150805115319.GB25784@node.dhcp.inet.fi>
References: <20150728171500.GA2871@www.outflux.net>
	<20150728143504.5aa996ba5955522a19c2d5f1@linux-foundation.org>
	<20150728221111.GA23391@node.dhcp.inet.fi>
	<20150805113854.GA9110@gmail.com>
	<20150805115319.GB25784@node.dhcp.inet.fi>
Date: Wed, 5 Aug 2015 06:13:22 -0700
X-Google-Sender-Auth: CxIY8JU6MfXTl0tmj-yPXTtpt74
Message-ID: <CAFoVXjjoV+fhEA1iwWasVa0UgHZjXq9uzGS0LqQmFwMzCwkq_A@mail.gmail.com>
Subject: Re: [PATCH] user_ns: use correct check for single-threadedness
From: Ricky Zhou <ricky@rzhou.org>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Ingo Molnar <mingo@kernel.org>, Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Oleg Nesterov <oleg@redhat.com>, David Howells <dhowells@redhat.com>,
        linux-kernel@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Rik van Riel <riel@redhat.com>,
        Vladimir Davydov <vdavydov@parallels.com>,
        Ricky Zhou <rickyz@chromium.org>, Julien Tinnes <jln@google.com>
Content-Type: text/plain; charset=UTF-8
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Wed, Aug 5, 2015 at 4:53 AM, Kirill A. Shutemov <kirill@shutemov.name> wrote:
>> 'only' if it's multi-threaded, i.e. when some workload cares so much about
>> performance that it uses multiple threads?
>>
>> Can you see the contradiction there?
>
> I can. man 2 unshare:
>
>         CLONE_NEWUSER requires that the calling process  is  not  threaded;
>
> The workload cares so much about performance that it ignores API
> requirements. Some slow down looks like a fair price to me.
To be fair, the entire reason for this patch is that the slow path
(mm_users > 1) can happen even when the process is single-threaded. I
was concerned about how expensive current_is_single_threaded looked as
well, but didn't see any lightweight alternatives short of adding a
field to mm_struct. Do folks think it's worth going down that route
instead?

Thanks,
Ricky