RFR: 8367317: ZGC: ZVirtualMemoryReserver::force_reserve_discontiguous arithmetic underflow [v3]

[Axel Boldt-Christmas]

> ZVirtualMemoryReserver::force_reserve_discontiguous is a debug only API used to simulate and test a discontinuous heap. The loop variable `end` is incorrectly checked which may lead to an underflow.
> 
> In our normal testing we never observe this as the address space is always available. We observed this issue when running a hardened build (like ASAN) which had already mmapped the address space we tried to reserve.
> 
> Rather than always jump two reserve sizes in its search, it searches every reserve size, moving an extra reserve size every time it succeeds with a reservation (so that there always is a gap).  Searching in reserve sized steps is not really a requirement here, we could have just used a granule sized step.

Axel Boldt-Christmas has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Merge tag 'jdk-26+18' into JDK-8367317
   
   Added tag jdk-26+18 for changeset 5251405c
 - Merge tag 'jdk-26+17' into JDK-8367317
   
   Added tag jdk-26+17 for changeset 2aafda19
 - 8367317: ZGC: ZVirtualMemoryReserver::force_reserve_discontiguous arithmetic underflow

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Changes:
  - all: https://git.openjdk.org/jdk/pull/27207/files
  - new: https://git.openjdk.org/jdk/pull/27207/files/6c77150f..0068e956

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27207&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27207&range=01-02

  Stats: 11978 lines in 389 files changed: 6399 ins; 3203 del; 2376 mod
  Patch: https://git.openjdk.org/jdk/pull/27207.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27207/head:pull/27207

PR: https://git.openjdk.org/jdk/pull/27207