RFR(xxs): 8187230: [aix] Leave OS guard page size at default for non-java threads instead of explicitly setting it

[Thomas Stüfe]

Hi all,

may I please have a review for this small change:

Bug:
https://bugs.openjdk.java.net/browse/JDK-8187230

Webrev:
http://cr.openjdk.java.net/~stuefe/webrevs/8187230-aix-
leave-os-guard-page-size-at-default-for-non-java-threads/webrev.00/webrev/

The change is very subtle.

Before, we would set the OS guard page size for every thread - for java
threads disable them, for non-java threads we'd set them to 4K.

Now, we still disable them for java threads but leave them at the OS
default size for non-java threads.

The really important part is the disabling of OS guard pages for java
threads, where we have a VM guard pages in place and do not want to spend
more memory on OS guards. We do not really care for the exact size of the
OS guard pages for non-java threads, and therefore should not set it - we
should leave the size in place the OS deems sufficient. That also spares us
the complexity of handling the thread stack page size, which on AIX may be
different from os::vm_page_size().

Thank you and Kind Regards, Thomas