RFR: 8326820: Metadata artificially kept alive

[Axel Boldt-Christmas]

On Tue, 18 Jun 2024 13:33:55 GMT, Erik Österlund <eosterlund at openjdk.org> wrote:

>> ClassLoaderDataGraph provides APIs for walking different metadata. All the iterators which are not designed to be used by the GC also keep the holder of the CLDs alive and by extensions keeps all metadata alive. This is problematic for concurrent GC as it keeps otherwise unreachable classes from being unloaded and the respective metadata freed. 
>> 
>> This patch changes the default iteration behaviour to not keep the holder alive, with the exception of `loaded_classes_do` (renamed `loaded_classes_do_keepalive`) and `modules_do` (renamed `modules_do_keepalive`) which is used by jvmti APIs that requires that the holder is kept alive.
>> 
>> All other uses consumes all the metadata it queries during its safepoint or before releasing the `ClassLoaderDataGraph_lock`. 
>> 
>> Before this change some jcmd, new jfr chunks and some jfr events, all of which consumed these APIs, could cause class unloading to not occur. 
>> 
>> Been running our internal stress test in an even more stressful mode which without this patch reproduces the metaspace OOME [JDK-8326005](https://bugs.openjdk.org/browse/JDK-8326005) consistently within a few hours. And after this patch it does not.
>> 
>> Currently running tier1-tier8 testing.
>
> src/hotspot/share/prims/jvmtiEnvBase.cpp line 2342:
> 
>> 2340: 
>> 2341:   // Iterate over all the modules loaded to the system.
>> 2342:   ClassLoaderDataGraph::modules_do_keepalive(&do_module);
> 
> Looks like this code exposes an OopHandle backed by the CLD handle area, which isn't a strong root that the GC will start tracing from. So it would seem that we need to keep these oops alive somehow.

`ClassLoaderDataGraph:: modules_do_keepalive ` is a `keepalive` iteration. That is it will load the holder of the CLD.
https://github.com/openjdk/jdk/blob/08366b1244775e5892bbbb184660821e8774f37a/src/hotspot/share/classfile/classLoaderDataGraph.cpp#L302-L310

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PR Review Comment: https://git.openjdk.org/jdk/pull/19769#discussion_r1644739284