RFR (S): 8200385: Eagerly reclaimed humongous objects leave mark in prev bitmap

[sangheon.kim]

Hi Thomas,

On 04/06/2018 10:58 AM, Thomas Schatzl wrote:
> Hi,
>
>    could a second person look at this please?
>
> Thanks,
>    Thomas
>
> On Thu, 2018-03-29 at 11:12 +0200, Thomas Schatzl wrote:
>> Hi all,
>>
>>    can I have reviews for this small fix to a benign bug, that is I
>> haven't seen any actual product failure from it but some very rare
>> test
>> failures, where when we eagerly reclaim humongous objects we leave a
>> mark on the prev bitmap in some cases?
>>
>> The suggested fix is to always look at the prev bitmap and clear it,
>> and if needed also clear potential marks in the next bitmap.
>>
>> To make the failure appear basically 100% in that test, I added a
>> simple assert after reclaiming the humongous object.
>>
>> With the fix, this failure goes away completely.
>>
>> Note that this is more a "data structure hygiene" fix - the stray
>> mark
>> on the prev bitmap will be automatically cleared after switching the
>> bitmaps at cleanup and preparing that bitmap for the next mark.
>>
>> CR:
>> https://bugs.openjdk.java.net/browse/JDK-8200385
>> Webrev:
>> http://cr.openjdk.java.net/~tschatzl/8200385/webrev/
Webrev.1 looks good.

Thanks,
Sangheon


>> Testing:
>> local testing with existing tests
>> (gc/g1/TestEagerReclaimHumongousRegionsClearMarkBits.java)
>>
>> Thanks,
>>    Thomas