RFR: 8337217: Port VirtualMemoryTracker to use VMATree [v33]

[Afshin Zafari]

On Mon, 3 Mar 2025 17:20:01 GMT, Gerard Ziemski <gziemski at openjdk.org> wrote:

>> Afshin Zafari has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   style, some cleanup, VMT and regionsTree circular dep resolved
>
> src/hotspot/share/nmt/regionsTree.cpp line 32:
> 
>> 30: VMATree::SummaryDiff RegionsTree::uncommit_region(address addr, size_t size) {
>> 31:   return uncommit_mapping((VMATree::position)addr, size, make_region_data(NativeCallStack::empty_stack(), mtNone));
>> 32: }
> 
> Would it be helpful here if we were to add a new tag, that would mark this uncommitted region somehow different than mtNone (to mark it that it used to be used, but now it's not, which is different from never used region)?

An uncommitted region has implicitly the same tag as its containing reserved region:

<-------------Reserved Region, mtXXX---------->
<----C1----><..U1...><---C2--><..U2..><---C3-->

C1-C3 are committed regions with tag `mtXXX`
U1 and U2 are uncommitted and enclosed by a `mtXXX` tag region.

Do you have any specific use-case for this?

---
If you were thinking about *release_mapping* instead, then I can say that the released region will be sooner or later reserved by another memtag. For example, an mtStack region is released and immediately reserved by mtGC. In other words, any memtag on a released region would be overwritten by other uses of the same region.

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/20425#discussion_r1979035962