linux-kernel - Re: [PATCH 2/2] cgroup: add xattr support

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Message-ID: <20120117175322.GC6762@google.com>
Date:	Tue, 17 Jan 2012 09:53:22 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Li Zefan <lizf@...fujitsu.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Cgroups <cgroups@...r.kernel.org>,
	Lennart Poettering <mzxreary@...inter.de>,
	Kay Sievers <kay.sievers@...y.org>
Subject: Re: [PATCH 2/2] cgroup: add xattr support

Hello,

On Mon, Jan 16, 2012 at 04:07:05PM +0800, Li Zefan wrote:
> This is one of the items in the plumber's wish list.
> 
> For use cases:
> 
> >> What would the use case be for this?
> >
> > Attaching meta information to services, in an easily discoverable
> > way. For example, in systemd we create one cgroup for each service, and
> > could then store data like the main pid of the specific service as an
> > xattr on the cgroup itself. That way we'd have almost all service state
> > in the cgroupfs, which would make it possible to terminate systemd and
> > later restart it without losing any state information. But there's more:
> > for example, some very peculiar services cannot be terminated on
> > shutdown (i.e. fakeraid DM stuff) and it would be really nice if the
> > services in question could just mark that on their cgroup, by setting an
> > xattr. On the more desktopy side of things there are other
> > possibilities: for example there are plans defining what an application
> > is along the lines of a cgroup (i.e. an app being a collection of
> > processes). With xattrs one could then attach an icon or human readable
> > program name on the cgroup.
> >
> > The key idea is that this would allow attaching runtime meta information
> > to cgroups and everything they model (services, apps, vms), that doesn't
> > need any complex userspace infrastructure, has good access control
> > (i.e. because the file system enforces that anyway, and there's the
> > "trusted." xattr namespace), notifications (inotify), and can easily be
> > shared among applications.
> >
> > Lennart
> 
> Signed-off-by: Li Zefan <lizf@...fujitsu.com>

Ummm... I don't feel too good about this.  Why can't those extra
information be kept somewhere else?  Overloading cgroupfs as storage
for essentially opaque userland information doesn't seem like a good
idea to me.

Thanks.

-- 
tejun
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