linux-kernel - Re: [PATCH] [RFC] mnt: add ability to clone mntns starting with the current root

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Message-ID: <CALCETrX4XrgbQNZZa7=1009KqhJ2gT+VBUkC15+59K9yEiTSbQ@mail.gmail.com>
Date:	Wed, 8 Oct 2014 08:35:22 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Andrew Vagin <avagin@...allels.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andrey Vagin <avagin@...nvz.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	Andrey Vagin <avagin@...il.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Rob Landley <rob@...dley.net>
Subject: Re: [PATCH] [RFC] mnt: add ability to clone mntns starting with the
 current root

On Wed, Oct 8, 2014 at 4:08 AM, Andrew Vagin <avagin@...allels.com> wrote:
> On Tue, Oct 07, 2014 at 01:45:22PM -0700, Eric W. Biederman wrote:
>> Andrey Vagin <avagin@...nvz.org> writes:
>>
>> > From: Andrey Vagin <avagin@...il.com>
>> >
>> > Currently when we create a new container with a separate root,
>> > we need to clone the current mount namespace with all mounts and then
>> > clean up it by using pivot_root(). A big part of mountpoints are cloned
>> > only to be umounted.
>>
>> Is the motivation performance?  Because if that is the motivation we
>> need numbers.
>
> The major motivation to create a clean mount namespace which contains
> only required mounts.
>
> Now you want to convince us that there is nothing wrong if we use
> userns, because all inherited mounts are locked. My point is that all
> useless mounts should be umounted.  If the current root isn't on rootfs,
> pivot_root() allows us to umount all useless points. But pivot_root()
> doesn't work, if the current root is on rootfs. How can we umount
> useless points in this case?
>
> Maybe we want to say that rootfs should not be used if we are going to
> create containers...
>

Could we have an extra rootfs-like fs that is always completely empty,
doesn't allow any writes, and can sit at the bottom of container
namespace hierarchies?  If so, and if we add a new syscall that's like
pivot_root (or unshare) but prunes the hierarchy, then we could switch
to that rootfs then.

> Thanks,
> Andrew
>
> --
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-- 
Andy Lutomirski
AMA Capital Management, LLC
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