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

[Afshin Zafari]

On Mon, 3 Mar 2025 17:10:40 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 28:
> 
>> 26: VMATree::SummaryDiff RegionsTree::commit_region(address addr, size_t size, const NativeCallStack& stack) {
>> 27:   return commit_mapping((VMATree::position)addr, size, make_region_data(stack, mtNone), /*use tag inplace*/ true);
>> 28: }
> 
> `RegionsTree::commit_region` is called by 
> 
>  ```
>  static inline void record_virtual_memory_reserve_and_commit(void* addr, size_t size,
>     const NativeCallStack& stack, MemTag mem_tag = mtNone) {
> 
> 
> which has mem_tag, so we could in theory use it and pass it down? Then we could avoid the complicated "use_tag_inplace" parameter handling?
> 
> Not sure if this is possible in all cases. Is that why we have the need for "use_tag_inplace"?

I have to separate the concerns as follow:
1. In `record_..._reserve_and_commit` the `mem_tag` is available since it is needed for `reserve`. So, you are right, we can use the tag and pass it down. The change set would be:
 `VMT::Instance::add_committed_region(addr, size, stack, MemTag = mtNone)`
 `VMT::add_committed_region(addr, size, stack, MemTag = mtNone)`
 `RegionsTree::commit_region(addr, size, stack, MemTag = mtNone)`
3. Even if we do so, `use_tag_in_place` is needed in `commit` because the `os::commit_memmory` family do not pass mem_tag down. There is already an abandoned PR which tried to add MemTag param to this family. It was reviewed as not necessary.
4. In addition, the `VMATree::register_mapping` has a mandatory MemTag param (in its MetaData param) which enforces the caller to pass down an specific MemTag. If we don't use `use_tag_inplace`, for every commit of region $[ base, end)$ we have to find the enclosing reserved region $[ A, B)$ where $A \le base < end \le B$ and get its mem_tag and then pass it down to `VMATree::register_mapping`. Finding the enclosing region could be too expensive and is preferred to be avoided. 

I can implement the case 1 above if it is preferred.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20425#discussion_r1978991855