RFR: 8252514: ZGC: Physical memory splitting is broken

[Per Liden]

Looks good!

/Per

On 8/31/20 1:51 PM, Stefan Karlsson wrote:
> Hi all,
> 
> Please review this patch to fix physical memory splitting in ZGC.
> 
> https://cr.openjdk.java.net/~stefank/8252514/webrev.01/
> https://bugs.openjdk.java.net/browse/JDK-8252514
> 
> JDK-8247759 changed the implementation to use a GrowableArray. One part 
> of that patch cached the value of _segments.at(index) inside 
> commit_segment and uncommit_segment. This is incorrect, since resizing 
> the array causes the backing memory to be deallocated, causing the the 
> local copy to point to garbage.
> 
> I've reverted that change, and now always call _segments.at(index) instead.
> 
> This was found while experimenting with physical memory allocations that 
> were likely to fail. I've verified that the failures stops reproducing 
> with this patch.
> 
> Thanks,
> StefanK