linux-kernel - Re: [PATCH 0/5 RFC] Add an interface to discover relationships between namespaces

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Date:	Mon, 25 Jul 2016 19:54:56 -0700
From:	Andrew Vagin <avagin@...tuozzo.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
	Andrey Vagin <avagin@...nvz.org>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	"criu@...nvz.org" <criu@...nvz.org>,
	Linux API <linux-api@...r.kernel.org>,
	Linux Containers <containers@...ts.linux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH 0/5 RFC] Add an interface to discover relationships
 between namespaces

On Mon, Jul 25, 2016 at 09:59:43AM -0500, Eric W. Biederman wrote:
> "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com> writes:

[snip]

> [snip]
> >>> So, from my point of view, the important piece that was missing from
> >>> your commit message was the note to use readlink("/proc/self/fd/%d")
> >>> on the returned FDs. I think that detail needs to be part of the
> >>> commit message (and also the man page text). I think it even be
> >>> helpful to include the above program as part of the commit message:
> >>> it helps people more quickly grasp the API.
> >>
> >> Please, please make the standard way to compare these things fstat.
> >> That is much less magic than a symlink, and a little more future proof.
> >> Possibly even kcmp.

I like the idea to use kcmp to compare namespaces. I am going to add this
functionality to kcmp and describe all these in the man page.

> >
> > As in fstat() to get the st_ino field, right?
> 
> Both the st_ino and st_dev fields.
> 
> The most likely change to support checkpoint/restart in the future is to
> preserve st_ino across migrations and instantiate a different instance
> of nsfs to hold the inode numbers from the previous machine.

It sounds tricky. BTW: Actually this is not only one places where we have
this sort of problem. For example, now mount id-s are not preserved when
a container is migrated. The same problem is applied to tmpfs, where
inode numbers are not preserved for files. 

> 
> We would need to handle the preservation carefully or else there is
> a chance that two namespace file descriptors (collected from different
> sources) with different st_dev and st_ino fields may actuall refer to
> the same object.
> 
> Which is a long way of saying we have the st_dev field please use it,
> it may matter at some point.
> 
> Eric