[jdk20] RFR: 8298084: Memory leak in Method::build_profiling_method_data [v3]

[David Holmes]

On Tue, 13 Dec 2022 00:56:30 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> src/hotspot/share/oops/instanceKlass.cpp line 2675:
>> 
>>> 2673:   if (release_sub_metadata) {
>>> 2674:     methods_do(method_release_C_heap_structures);
>>> 2675:   }
>> 
>> I don't understand this aspect of the change. It seems unrelated to the missing destructor issue and seems to cause a significant change in behaviour as we will no longer call `methods_do` when called from `deallocate_contents` with the renamed `false` argument.
>
> This boolean prevents us from calling the MethodData destructor twice and releasing MethodData C heap memory twice.  InstanceKlass::deallocate_contents calls release_C_heap_structures with it false, and then calls through the Metadata pointer hierarchy to MetadataFactory::free_metadata, which calls the MethodData destructor and MethodData::deallocate_contents which calls MethodData::release_C_heap_structures().   This is the delegation scheme for releasing metadata between InstanceKlass and ConstantPool.
> 
> InstanceKlass::release_C_heap_structures() is also called during class unloading and must walk the whole metadata, releasing C heap memory.  It's passed true. It doesn't go through deallocate_contents because the metadata is freed together at the same time with the metaspace that contains it.

Thanks Coleen I see now how the code has been shuffled around:

Old code:

InstanceKlass::deallocate_contents
-> release_C_heap_structures(/* release_sub_metadata */ false);
  -> methods_do(method_release_C_heap_structures);
    -> m->release_C_heap_structures
      -> method_data()->~MethodData();
-> deallocate_methods(loader_data, methods());
  -> MetadataFactory::free_metadata(loader_data, method);

New code:

InstanceKlass::deallocate_contents
-> release_C_heap_structures(/* release_sub_metadata */ false);
-> deallocate_methods(loader_data, methods());
  -> MetadataFactory::free_metadata(loader_data, method);
    -> method_data()->~MethodData();

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

PR: https://git.openjdk.org/jdk20/pull/13