Namespaces exhausted CLONE_XXX bits problem

[Pavel Emelyanov <xemul-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org>]

Hi, guys!

I started looking at PTYs/TTYs/Console to make the appropriate
namespace and suddenly remembered that we have already
exhausted all the CLONE_ bits in 32-bit mask.

So, I recalled the discussions we had and saw the following 
proposals of how to track this problem (with their disadvantages):

1. make the clone2 system call with 64-bit mask
   - this is a new system call
2. re-use CLONE_STOPPED
   - this will give us only one bit
3. merge existing bits into one
   - we lose the ability to create them separately
4. implement a sys_unshare_ns system call with 64bit/arbitrary mask
   - this is anew system call
   - this will bring some dissymmetry between namespaces
5. use sys_indirect
   - this one is not in even -mm tree yet and it's questionable
     whether it will be at all

I have one more suggestion:

6. re-use bits, that don't make sense in sys_unshare (e.g.
   CLONE_STOPPED, CLONE_PARENT_SETTID, CLONE_VFORK etc)
   This will give us ~16 new bits, but this will look not very nice.

What do you think about all of this?

Thanks,
Pavel