RFR (XS) 8141422: G1 eager reclaim card dirtying may dirty outide of allocated objects

Mikael Gerdin mikael.gerdin at oracle.com
Wed Nov 4 13:20:35 UTC 2015

Hi all,

We've discovered a small issue with the card dirtying used to support 
eager reclaim of humongous objects.

The problem is that when remembered set iteration yields a card which 
does not correspond to the allocated part of a region then that cards 
should be ignored. Currently the code only detects this situation when 
the ignored card covers an otherwise free region but this check needs to 
be extended to also check if the card corresponds to unallocated memory 
in an otherwise used region.
An example of this situation is that the card corresponds to a part of 
memory above _top in a humongous region and this is what we are seeing 
in some test cases.

My suggested fix is to check "is_in_closed_subset" which will look up 
the corresponding region and verify that the address is within the 
allocated bounds of that region.

Bug: https://bugs.openjdk.java.net/browse/JDK-8141422
Webrev: http://cr.openjdk.java.net/~mgerdin/8141422/webrev.0/
Testing: JPRT


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