RFR: 8272807: Permit use of memory concurrent with pretouch [v4]

[Kim Barrett]

> [Trying again, after a couple previous proposals.]
> 
> Please review this change to os::pretouch_memory to permit use of the memory
> being pretouched concurrently with the pretouch operation.  This will allow
> future changes that can make use of such overlap.
> 
> Testing:
> mach5 tier1-3 with -XX:+AlwaysPreTouch.
> 
> Verified no performance impact by comparing startup times with Epsilon GC and
> -XX:+AlwaysPretouch on a large enough heap that the pretouch takes a
> significant amount of time.  Tested on both linux-x64 and linux-aarch64.
> 
> None of these test concurrent use; that will wait for future changes that use
> the new feature.

Kim Barrett 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 four additional commits since the last revision:

 - Merge branch 'master' into conc-pretouch2
 - change loop termination test
 - assert range is valid
 - pretouch allowing concurrent access

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7343/files
  - new: https://git.openjdk.java.net/jdk/pull/7343/files/2e4e66a0..7fd0be0b

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7343&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7343&range=02-03

  Stats: 5280 lines in 106 files changed: 4018 ins; 743 del; 519 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7343.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7343/head:pull/7343

PR: https://git.openjdk.java.net/jdk/pull/7343