RFR (S) 8220162: Shenandoah should not commit HugeTLBFS memory

[Roman Kennke]

Hmm... I find the designation 'special' not very useful, but fail to 
come up with anything better. I guess it's ok. Reviewed. (Unless you 
find a better name for it.. ;-) )

Roman

> Bug:
>    https://bugs.openjdk.java.net/browse/JDK-8220162
> 
> Fix:
>    http://cr.openjdk.java.net/~shade/8220162/webrev.02/
> 
> This is a funny bug: technically, +UseLargePages works and allocates heap in hugetlbfs, but it
> _also_ commits stuff in usual memory after that. Which was found by allocating large enough
> hugetlbfs so that double-allocation fails with OOME right away. The apparent way out is to check if
> we have reserved a "special" space, and skip commits then.
> 
> Unfortunately, again, constructing a reliable regression test proved hard, so I had to resort to
> manual testing.
> 
> Testing: adhoc runs with HugeTLBFS enabled, hotspot_gc_shenandoah {with and without -XX:+UseLargePages}
> 
> Thanks,
> -Aleksey
>