NMT and adjacent reserved regions

[Thomas Stüfe]

Hi,

I am currently working at a fix for JDK-8243535
<https://bugs.openjdk.java.net/browse/JDK-8243535> (NMT does not handle
os::split_reserved_space()).

I was looking at how NMT treats adjacent reserved regions. Do we collapse
them in NMT? The code here

http://hg.openjdk.java.net/jdk/jdk/file/a44396903fd8/src/hotspot/share/services/virtualMemoryTracker.cpp#l351

seems to suggest we do.

However I believe we never enter this case since the region was obtained
using _reserved_regions->find(rgn); which only returns overlapping regions,
excluding neighbors.

So IIUC currently we do not collapse adjacent regions. I think this is also
the correct behavior. Is above posted code snippet an overlook then?

Or do we want to collapse regions? I think this makes not much sense
though, since neighboring reservations may and do come from different
callers with different memory flags.

Thanks for clarifying,

Thomas