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

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Date:	Thu, 28 Jul 2016 21:00:32 +0200
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	mtk.manpages@...il.com, Andrew Vagin <avagin@...tuozzo.com>,
	Andrey Vagin <avagin@...nvz.org>,
	"Serge E. Hallyn" <serge@...lyn.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

Hi Eric,

On 07/28/2016 02:56 PM, Eric W. Biederman wrote:
> "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com> writes:
>
>> On 07/26/2016 10:39 PM, Andrew Vagin wrote:
>>> On Tue, Jul 26, 2016 at 09:17:31PM +0200, Michael Kerrisk (man-pages) wrote:
>
>>> If we want to compare two file descriptors of the current process,
>>> it is one of cases for which kcmp can be used. We can call kcmp to
>>> compare two namespaces which are opened in other processes.
>>
>> Is there really a use case there? I assume we're talking about the
>> scenario where a process in one namespace opens a /proc/PID/ns/*
>> file descriptor and passes that FD to another process via a UNIX
>> domain socket. Is that correct?
>>
>> So, supposing that we want to build a map of the relationships
>> between namespaces using the proposed kcmp() API, and there are
>> say N namespaces? Does this mena we make (N * (N-1) / 2) calls
>> to kcmp()?
>
> Potentially.  The numbers are small enough O(N^2) isn't fatal.

Define "small", please.

O(N^2) makes me nervous about what other use cases lurk out
there that may get bitten by this.

> Where kcmp shines is that it allows migration to happen.  Inode numbers
> to change (which they very much will today), and still have things work.


> We can keep it O(Nlog(N)) by taking advantage of not just the equality
> but the ordering relationship.  Although Ugh.

Yes, that sounds pretty ugly...

>One disadvantage of
> kcmp currently is that the way the ordering relationship is defined
> the order is not preserved over migration :(

So, does kcmp() fully solve the proble(s) at hand? It sounds like
not, if I understand your last point correctly.


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/