Integrated: 8253081: G1 fails on stale objects in archived module graph in Open Archive regions

[Thomas Schatzl]

On Wed, 11 Nov 2020 11:39:59 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

This pull request has now been integrated.

Changeset: d3095605
Author:    Thomas Schatzl <tschatzl at openjdk.org>
URL:       https://git.openjdk.java.net/jdk/commit/d3095605
Stats:     694 lines in 32 files changed: 464 ins; 110 del; 120 mod

8253081: G1 fails on stale objects in archived module graph in Open Archive regions

Change the handling of Open Archive areas, instead of assuming that everything in there is live always, a root containing references to all live root objects is provided. Adapt G1 to handle Open Archive regions as any other old region apart from never compacting or evacuating them.

Co-authored-by: Ioi Lam <iklam at openjdk.org>
Reviewed-by: kbarrett, sjohanss, redestad

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