[External] : Potential memory leak in Method::ensure_jmethod_ids

[Coleen Phillimore]

Hi Jiangli,

The jmethodID leak is intentional for backward compatibility. Looking 
back through the source code history, I don't find a bug that made 
leaking jmethodID objects required.  It seems like a useful change to 
clean them up under a product option.  When I measured the code we had, 
there weren't many jmethodID objects, but applications can differ.

Thanks,
Coleen


On 1/24/21 8:46 PM, Jiangli Zhou wrote:
> Hi,
>
> Developers reported a potential memory leak in 
> Method::ensure_jmethod_ids for a longer running application on JDK 11 
> (most likely the memory leak exists in newer JDK versions as well). 
> The allocated JNIMethodBlockNodes are never freed, which is the case 
> even for user defined class loaders after unloading. The daily leak is 
> ~20MiB - ~70MiB. Although the memory leak is not very large, it's 
> still good to eliminate it.
>
> The following comment in ClassLoaderData::~ClassLoaderData() 
> <https://urldefense.com/v3/__https://github.com/openjdk/jdk/blob/764111ff831096729eb1bcc69a54d92fde67b044/src/hotspot/share/classfile/classLoaderData.cpp*L705__;Iw!!GqivPVa7Brio!JFfqo4Dc12kzqeunN7e-UxlXKAKyj4OCzahIumZpuTrEhq3HOx82KsIu8nrrG6e4rqUN7w$> 
> indicates it's intentional:
>
>     // Clear all the JNI handles for methods
>     // These aren't deallocated and are going to look like a leak, but
>     that's
>     // needed because we can't really get rid of jmethodIDs because we
>     don't
>     // know when native code is going to stop using them. The spec
>     says that
>     // they're "invalid" but existing programs likely rely on their being
>     // NULL after class unloading.
>     if (_jmethod_ids != NULL) {
>       Method::clear_jmethod_ids(this);
>     }
>
>
> The change was introduced as part of the large permgen removal effort. 
> It's not too clear what are the side-effects (related to JVMTI agent?) 
> of deallocating the JNIMethodBlockNodes after unloading. Does anyone 
> know if there was a specific bug related?
>
> Thanks!
>
> Best,
> Jiangli