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

[Andy Lutomirski]

On Wed, Oct 8, 2014 at 4:08 AM, Andrew Vagin <avagin@xxxxxxxxxxxxx> wrote:
> On Tue, Oct 07, 2014 at 01:45:22PM -0700, Eric W. Biederman wrote:
>> Andrey Vagin <avagin@xxxxxxxxxx> writes:
>>
>> > From: Andrey Vagin <avagin@xxxxxxxxx>
>> >
>> > 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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