RFR: 8253081: G1 fails on stale objects in archived module graph in Open Archive regions [v4]

[Kim Barrett]

On Tue, 17 Nov 2020 10:55:19 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> Hi all,
>> 
>>   can I have reviews for this change that changes the way how archive regions are managed in general and specifically by the G1 collector, fixing the crashes caused by adding the module graph into the archive in [JDK-8244778](https://bugs.openjdk.java.net/browse/JDK-8244778)?
>> 
>> Previously before the JDK-8244778 change, archived objects could always be assumed as live, and so the G1 collector did so, not caring about the archive region's contents at all. With JDK-8244778 however, archived objects could die, and keep stale references to objects outside of the archive regions, which obviously causes crashes when walking these objects.
>> 
>> With this change, open archive region contents are basically handled as any other objects; to support that, all open archive regions are now reachable via a single object array root. This hopefully also facilitates implementation in other collectors.
>> 
>> This allows us to remove quite a bit of special handling in G1 too; the only difference is that open archive regions will generally not be collected unless they are completely empty: we do want to profit from the sharing across VMs as much as possible.
>> 
>> Testing: tier1-5, one or two 6-8 runs
>> 
>> The appcds changes were done by @iklam. These changes are described in this document: https://wiki.openjdk.java.net/display/HotSpot/CDS+Archived+Heap+Improvements
>> 
>> Thanks,
>>   Thomas
>
> Thomas Schatzl has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - sjohanss review
>  - Remove code that "activates" dormant objects as now all active objects are reachable via the root object array

Marked as reviewed by kbarrett (Reviewer).

src/hotspot/share/memory/heapShared.cpp line 660:

> 658:     VM_Verify verify_op;
> 659:     VMThread::execute(&verify_op);
> 660:     if (!FLAG_IS_DEFAULT(VerifyArchivedFields)) {

Comment says "command line", so this should be FLAG_IS_CMDLINE rather than !FLAG_IS_DEFAULT.

src/hotspot/share/memory/heapShared.cpp line 662:

> 660:     if (!FLAG_IS_DEFAULT(VerifyArchivedFields)) {
> 661:       // If this -XX:+VerifyArchivedFields is specified on the command-line, do extra
> 662:       // checks.

s/this//

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PR: https://git.openjdk.java.net/jdk/pull/1163