RFR: JDK-8312492: Remove THP sanity checks at VM startup

[Thomas Stuefe]

When starting the JVM with -XX:+UseTransparentHugePages, we need to know if the OS supports THPs:

A) We could run on a super-old kernel (very unlikely, THPs are supported since 2.6.38 which is EOL since 2011)
B) We could run on a kernel built without THP support, e.g. an embedded device
C) We could run on a system with THPs disabled by the admin

The JVM does a little sanity test at startup: it mmaps an area the size of a huge page - but in small pages - then calls madvise(MADV_HUGEPAGE) on it. The JVM assumes if the madvise() worked, all is fine.

That sanity test is expensive and has little merit. 

It will not detect if THPs are disabled on the system (C) - since madvise reports success even then.

It is expensive since we allocate (reserve AND commit) an area, then signal khugepaged by advising to fold it into a huge page. That area could be large if the kernel supports 1 GB THP pages. We may end up committing 1 GB space at startup. And depending on the THP defrag settings, that allocation may have to wait for the khugepaged to allocate a 1 GB page. Even if it does not wait but relies on khugepaged to work concurrently its a bad idea.

The correct - and much cheaper - way is to check the proc fs whether THPs are enabled. We already do that since [JDK-8310233](https://bugs.openjdk.org/browse/JDK-8310233): "Fix THP detection on Linux". That check takes care of (A)(B)(C). Therefore we can remove the THP sanity check completely.

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Commit messages:
 - remove-thp-sanity-tests

Changes: https://git.openjdk.org/jdk/pull/14967/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14967&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8312492
  Stats: 37 lines in 2 files changed: 0 ins; 37 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/14967.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14967/head:pull/14967

PR: https://git.openjdk.org/jdk/pull/14967