8212205 (XS) VM asserts after CDS archive has been unmapped

Jiangli Zhou jiangli.zhou at oracle.com
Thu Oct 25 22:11:37 UTC 2018

On 10/25/18 1:49 PM, Ioi Lam wrote:

> MetaspaceObj::_shared_metaspace_top is a private field. 
> MetaspaceShared can modify the field because it's a friend class of 
> MetaspaceObj. I don't want to add more friend classes. Also, I think 
> FileMapInfo class is more like an internal class for MetaspaceShared 
> and other classes should avoid using it.
How about adding something like MetaspaceShared::clear_shared_metaspace 
(or MetaspaceShared::reset_shared_metaspace)? It could be called from 
MetaspaceShared::stop_sharing_and_unmap (or from metaspace.cpp 
directly). The reason I bring it up is because having two public 
stop_sharing_and_unmap() APIs is a bit confusing and 
FileMapInfo::stop_sharing_and_unmap() is no longer intended to be used 
by others for general cases. Or how about making 
FileMapInfo::stop_sharing_and_unmap() private (add MetaspaceShared as a 
friend to FileMapInfo)?

If you want to keep both public APIs, probably consider at least adding 
a comment warning about the use of FileMapInfo::stop_sharing_and_unmap()?

> Thanks
> - Ioi
> On 10/25/18 12:20 PM, Jiangli Zhou wrote:
>> Hi Ioi,
>> The fix looks good, but it's probably better to fix it in 
>> FileMapInfo::stop_sharing_and_unmap() directly?
>> Thanks,
>> Jiangli
>> On 10/25/18 11:22 AM, Ioi Lam wrote:
>>> https://bugs.openjdk.java.net/browse/JDK-8212205
>>> http://cr.openjdk.java.net/~iklam/jdk12/8212205-crash-after-cds-unmap.v01/ 
>>> This is a silly bug but it happens only rarely, when address space 
>>> randomization
>>> causes insufficient memory to be reservable at addresses required by 
>>> CDS. In this
>>> case, the CDS archive is unmapped, but we forget to clean up
>>> _shared_metaspace_{base,top}. As a result, the JVM will misidentify 
>>> metaspace
>>> objects allocated in certain address ranges to be "shared", in the 
>>> following function:
>>> class MetaspaceObj {
>>>   bool is_shared() const {
>>>     return (((void*)this) < _shared_metaspace_top && ((void*)this) 
>>> >= _shared_metaspace_base);
>>>   }
>>> }
>>> The fix is to reset those pointers after CDS has been unmapped.
>>> Thanks
>>> - Ioi

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