RFR: 8354358: ZGC: ZPartition::prime handle discontiguous reservations correctly [v3]

[Stefan Karlsson]

On Mon, 14 Apr 2025 12:53:18 GMT, Axel Boldt-Christmas <aboldtch at openjdk.org> wrote:

>> Prior to [JDK-8350441](https://bugs.openjdk.org/browse/JDK-8350441) the VM would not have started if we received a discontiguous heap reservation with all reservations smaller than the inital heap capacity. Now we crash because `ZPartition::prime` does not take this into account.
>> 
>> However in contrast to the page cache, the mapped cache makes it trivial to support this scenario. So I propose fixing `ZPartition::prime` to handle any discontiguous heap reservation.
>> 
>> Can be provoked in a debug build by using ZForceDiscontiguousHeapReservations > 16
>> `java -XX:+UseZGC -XX:ZForceDiscontiguousHeapReservations=17 -Xmx128m -Xms128m --version`
>> 
>> Currently running this through testing.
>
> Axel Boldt-Christmas has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update outdated TestZNMT.java comment

Marked as reviewed by stefank (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/24589#pullrequestreview-2764278721