forkbomb killer

Cleaned up and fixed unclear logics. and removed RFC.
Maybe this version is easy to be read.


When we see forkbomb, it tends can be a fatal one.

 When A user makes a forkbomb (and sometimes reaches ulimit....
   In this case, 
   - If the system is not in OOM, the admin may be able to kill all threads by
     hand..but forkbomb may be faster than pkill() by admin.
   - If the system is in OOM, the admin needs to reboot system.
     OOM killer is slow than forkbomb.

So, I think forkbomb killer is appreciated. It's better than reboot.

At implementing forkbomb killer, one of difficult case is like this

# forkbomb(){ forkbomb|forkbomb & } ; forkbomb

With this, parent tasks will exit() before the system goes under OOM.
So, it's difficult to know the whole image of forkbomb.

This patch introduce a subsystem to track mm's history and records it
even after the task exit. (It will be flushed periodically.)

I tested with several forkbomb cases and this patch seems work fine.

Maybe some more 'heuristics' can be added....but I think this simple
one works enough. Any comments are welcome.
Thanks,
-Kame


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