imho very good idea and we'll have more and more docs there...
(the only issue I see - it would be nice to have cgroups docs in the same place,
though cgroups are not about namespaces directly.)

Acked-By: Kirill Korotaev <dev@sw.ru>


Pavel Emelyanov wrote:
> Hi guys!
> 
> As you might have seen, recently there was some spontaneous 
> discussion about the namespaces-working-together problems.
> 
> Ted T'so proposed to create some document that describes what
> problems user may have when he/she creates some new namespace,
> but keeps others shared. I like this idea, so here's the draft
> with the problems I currently have in mind and can describe 
> somewhat audibly - the "namespaces compatibility list".
> 
> The Documentation/namespaces/ dir is about to contain more
> docs about the namespaces stuff (e.g. I'm going to prepare
> a doc about the pid namespaces, maybe Serge will want to
> write something about the user namespaces development, Eric
> may want to put some notes about the netns API and so on), 
> but currently there will be only one file.
> 
> What would you say about it?
> 
> Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
> 
> ---
> 
> diff --git a/Documentation/namespaces/compatibility-list.txt b/Documentation/namespaces/compatibility-list.txt
> new file mode 100644
> index 0000000..4be4a3c
> --- /dev/null
> +++ b/Documentation/namespaces/compatibility-list.txt
> @@ -0,0 +1,32 @@
> +	Namespaces compatibility list
> +
> +This document contains the information about the problems user
> +may have when creating tasks living in different namespaces.
> +
> +Here's the summary. This matrix shows the known problems, that
> +occur when tasks share some namespace (the columns) while living
> +in different other namespaces (the raws):
> +
> +	UTS	IPC	VFS	PID	User	Net
> +UTS
> +IPC			 1
> +VFS
> +PID		 1	 1
> +User			 2
> +Net
> +
> +1. Both the IPC and the PID namespaces provide IDs to address
> +   object inside the kernel. E.g. semaphore with ipcid or
> +   process group with pid.
> +
> +   In both cases, tasks shouldn't try telling this id to some
> +   other task living in different namespace vid shared filesystem
> +   or IPC shmem/message. The fact is that this ID is only valid
> +   within the namespace it was obtained in and may refer to some
> +   other object in another namespace.
> +
> +2. Intentionnaly, two equal user ids in different user namespaces
> +   should not be equal from the VFS point of view. In other
> +   words, user 10 in one user namespace shouldn't have the same
> +   access permissions to files, beloging to user 10 in another
> +   namespace. But currently this is not so.
> 
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> 

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