linux-kernel - Re: [PATCHv2 7/7] cgroup: mount cgroupns-root when inside non-init cgroupns

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Message-ID: <20141104155052.GA7027@mail.hallyn.com>
Date:	Tue, 4 Nov 2014 16:50:52 +0100
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Tejun Heo <tj@...nel.org>, Linux API <linux-api@...r.kernel.org>,
	Linux Containers <containers@...ts.linux-foundation.org>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	cgroups@...r.kernel.org, Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCHv2 7/7] cgroup: mount cgroupns-root when inside non-init
 cgroupns

Quoting Andy Lutomirski (luto@...capital.net):
> On Tue, Nov 4, 2014 at 5:46 AM, Tejun Heo <tj@...nel.org> wrote:
> > Hello, Aditya.
> >
> > On Mon, Nov 03, 2014 at 02:43:47PM -0800, Aditya Kali wrote:
> >> I agree that this is effectively bind-mounting, but doing this in kernel
> >> makes it really convenient for the userspace. The process that sets up the
> >> container doesn't need to care whether it should bind-mount cgroupfs inside
> >> the container or not. The tasks inside the container can mount cgroupfs on
> >> as-needed basis. The root container manager can simply unshare cgroupns and
> >> forget about the internal setup. I think this is useful just for the reason
> >> that it makes life much simpler for userspace.
> >
> > If it's okay to require userland to just do bind mounting, I'd be far
> > happier with that.  cgroup mount code is already overcomplicated
> > because of the dynamic matching of supers to mounts when it could just
> > have told userland to use bind mounting.  Doesn't the host side have
> > to set up some of the filesystem layouts anyway?  Does it really
> > matter that we require the host to set up cgroup hierarchy too?
> >
> 
> Sort of, but only sort of.
> 
> You can create a container by unsharing namespaces, mounting
> everything, and then calling pivot_root.  But this is unpleasant
> because of the strange way that pid namespaces work -- you generally
> have to fork first, so this gets tedious.  And it doesn't integrate
> well with things like fstab or other container-side configuration
> mechanisms.
> 
> It's nicer if you can unshare namespaces, mount the bare minimum,
> pivot_root, and let the contained software do as much setup as
> possible.

Also, the bind-mount requires the container manager to know where
the guest distro will want the cgroups mounted.

-serge
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