[Devel] Re: [patch -mm 08/17] nsproxy: add hashtable

[Cedric Le Goater]

Herbert Poetzl wrote:
> On Fri, Dec 08, 2006 at 01:57:38PM -0700, Eric W. Biederman wrote:
>> "Serge E. Hallyn" <serue at us.ibm.com> writes:
>>
>>> Quoting Eric W. Biederman (ebiederm at xmission.com):
>>>> clg at fr.ibm.com writes:
>>>>
>>>>> From: Cedric Le Goater <clg at fr.ibm.com>
>>>>>
>>>>> This patch adds a hashtable of nsproxy using the nsproxy as a key. 
>>>>> init_nsproxy is hashed at init with key 0. This is considered to be 
>>>>> the 'host' nsproxy.
>>>> NAK.  Which namespace do these ids live in?
> 
> well, I gave a similar answer in another email,
> so I fully agree with the NAK here ...

hmm, I wasn't that clear to me. OK, let's dig :)

>>>> It sounds like you are setting up to make the 'host' nsproxy
>>>> special and have special rules. That also sounds wrong.
>>>>
>>>> Even letting the concept of nsproxy escape to user space sounds
>>>> wrong. nsproxy is an internal space optimization. It's not struct
>>>> container and I don't think we want it to become that.
>>>>
>>>> Eric
>>> So would you advocate referring to containers just by the pid of
>>> a process containing the nsproxy, and letting userspace maintain
>>> a mapping of id's to containers through container create/enter
>>> commands? Or is there some other way you were thinking of doing
>>> this?
> 
>> There are two possible ways.
>> 1) Just use a process using the namespace.
>>    This is easiest to implement.
> 
>> 2) Have a struct pid reference in the namespace itself, 
>>    and probably an extra pointer in struct pid to find it.
>>    This is the most stable, because fork/exit won't affect 
>>    which pid you need to use.
> 
> while I agree that nsproxy is definitely the wrong
> point to tie a 'context' too, as it can contain a
> mixture of spaces from inside and outside a context,
> and it would require to forbid doing things like
> clone() with the space flags, both inside and outside
> a 'container' to allow to use them for actual vps
> applications, I think that we have to have some kind
> of handle to tie specific sets of namespaces too

this is nsproxy ... 
 
> that 'can' be an nsproxy or something different, but
> I'm absolutely unhappy with tying it to a process,

hmm, what do you mean ? nsproxy survives the death of any 
process. It's not tied to any process in particular. One
process creates it with an unshare but that's all.

the ->nsproxy in task_struct is a way to find it.

> as I already mentioned several times, that lightweight
> 'containers' do not use/have an init process, and no
> single process might survive the entire life span of
> that 'container' ...

I think there is a misunderstanding here. a 'container' 
or 'nsproxy' or what ever is a set of namespaces which
are not tied to a process. 

you can do that today on 2.6.19 with utsname. 

>> Beyond that yes it seems to make sense to let user space 
>> maintain any mapping of containers to ids.
> 
> I agree with that, but we need something to move
> around between the various spaces ...

the bind_ns syscall lets the user specify the mapping. this 
is not done by the kernel. 

I had to introduce some rules, like giving more capabilities to 
some processes, but that can be changed. For the 
moment, they have to live in "init_proxy".

> for example, Linux-VServer ties the namespaces to
> the context structure (atm) which allows userspace
> to set and enter specific spaces of a guest context
> (I assume OpenVZ does similar)

What's the big difference with nsproxy ? 

C.
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