RFR: 8017629: G1: UseSHM in combination with a G1HeapRegionSize > os::large_page_size() falls back to use small pages

[Stefan Karlsson]

Hi all,

Please review this patch to enable SHM large page allocations even when 
the requested alignment is larger than os::large_page_size().

http://cr.openjdk.java.net/~stefank/8017629/webrev.01
https://bugs.openjdk.java.net/browse/JDK-8017629

G1 is affected by this bug since it requires the heap to start at an 
address that is aligned with the heap region size. The patch fixes this 
by changing the UseSHM large pages allocation code. First, virtual 
memory with correct alignment is pre-reserved and then the large pages 
are attached to this memory area.

Tested with vm.gc.testlist and ExecuteInternaVMTests

Thanks,
StefanK