From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754637Ab1L2QXW (ORCPT ); Thu, 29 Dec 2011 11:23:22 -0500 Received: from mail-tul01m020-f174.google.com ([209.85.214.174]:60203 "EHLO mail-tul01m020-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754227Ab1L2QXT (ORCPT ); Thu, 29 Dec 2011 11:23:19 -0500 Date: Thu, 29 Dec 2011 08:23:13 -0800 From: Tejun Heo To: Li Zefan Cc: LKML , Cgroups Subject: Re: [PATCH] cgroup: fix to allow mounting a hierarchy by name Message-ID: <20111229162313.GD3516@google.com> References: <4EF9291D.7030208@cn.fujitsu.com> <20111227163535.GA17712@google.com> <4EFAB2E2.4040008@cn.fujitsu.com> <20111228163631.GN17712@google.com> <4EFBD55E.5030106@cn.fujitsu.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4EFBD55E.5030106@cn.fujitsu.com> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello, On Thu, Dec 29, 2011 at 10:50:06AM +0800, Li Zefan wrote: > Tejun Heo wrote: > > Hello, Li. > > > > On Wed, Dec 28, 2011 at 02:10:42PM +0800, Li Zefan wrote: > >> The "name" option was introduced along with the "none" option, so we > >> can distinguish between different cgroup hierarchies which have no > >> bound subsystems, like this: > >> > >> # mount -t cgroup -o none,name=hier1 xxx /cgroup1 > >> # mount -t cgroup -o none,name=hier2 xxx /cgroup2 > >> > >> As the name is unique, we have this "mount by hierarchy name" feature. > > > > I could be missing something but does that add anything other than > > naming convenience? > > The name option is necessary, otherwise how can we mount hierarchies > as shown in the above example? I don't think mounting itself would be a problem. We don't need name option to create multiple tmpfs instances, right? The problem is referencing to them after they're created. Filesystems generally don't need such identifier because, once they're created, they can be referenced by their mount points. I'm still not very familiar with different corners of cgroup, so it's entirely possible that I'm missing something. If I am, please point me to it. > > If it's a redundant feature which has been broken over a year without > > anyone complaining, it really doesn't need to exist. It might not > > save a lot of code but would save some WTH moments. > > > > The redundant feature is mouting existing hierarchies by specifying name > only, and the cleanup patch I sent has this feature removed in effect. > > kernel/cgroup.c | 15 +++++++-------- > 1 files changed, 7 insertions(+), 8 deletions(-) > > This is why I'm not so keen to remove the feature. Code reduction is definitely a plus and I don't want to remove a useful feature either, but an unusual redundant feature without necessity is confusing / misleading even if it doesn't necessasrily add a lot of code complexity. Also, I at least want to understand why it's actually necessary before applying the patches. :) Thanks. -- tejun