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)" [off-list ref] writes:

quoted

On 07/26/2016 10:39 PM, Andrew Vagin wrote:

quoted

On Tue, Jul 26, 2016 at 09:17:31PM +0200, Michael Kerrisk (man-pages) wrote:

quoted

quoted

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/