RFR: 8315923: pretouch_memory by atomic-add-0 fragments huge pages unexpectedly [v2]

[Liming Liu]

> As described at [JDK-8315923](https://bugs.openjdk.org/browse/JDK-8315923), this patch uses madvise with MADV_POPULATE_WRITE to pretouch memory when supported (since kernel 5.14).
> 
> Ran the newly added jtreg test on 64c Neoverse-N1 machines with kernel 4.18, 5.13 and 6.1, and observed that transparent huge pages formed right after pretouch on kernel 6.1. Recorded the time spent on the test in *seconds* with `VERBOSE=time` as the table below, and got that the patch takes improvements when the system call is supported, while does not hurt if not supported:
> 
> <table>
>   <tr>
>     <th>Kernel</th>
>     <th colspan="2"><tt>-XX:-TransparentHugePages</tt></th>
>     <th colspan="2"><tt>-XX:+TransparentHugePages</tt></th>
>   </tr>
>   <tr><td></td><td>Unpatched</td><td>Patched</td><td>Unpatched</td><td>Patched</td></tr>
>   <tr><td>4.18</td><td>11.30</td><td>11.30</td><td>0.25</td><td>0.25</td></tr>
>   <tr><td>5.13</td><td>0.22</td><td>0.22</td><td>3.42</td><td>3.42</td></tr>
>   <tr><td>6.1</td><td>0.27</td><td>0.33</td><td>3.54</td><td>0.33</td></tr>
> </table>

Liming Liu has updated the pull request incrementally with six additional commits since the last revision:

 - Use pointer_delta to calculate the distance
 - Add a sanity check for MADV_POPULATE_WRITE
 - Fix grammar issues
 - Add assertions in pretouch_memory_common
 - Cuddle ptr-operators with types to meet HotSpot style
 - Remove the Unnecessary line continue character

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/15781/files
  - new: https://git.openjdk.org/jdk/pull/15781/files/1d1c8349..51533b9a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15781&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15781&range=00-01

  Stats: 20 lines in 7 files changed: 7 ins; 0 del; 13 mod
  Patch: https://git.openjdk.org/jdk/pull/15781.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15781/head:pull/15781

PR: https://git.openjdk.org/jdk/pull/15781