RFR(xs): 8210068: Unsafe.allocateMemory() should not round up requested memory size

[Thomas Stüfe]

Hi all,

I am cleaning up my bug queue for 12.

may I please have reviews for this small fix:

Bug: https://bugs.openjdk.java.net/browse/JDK-8210068
cr: http://cr.openjdk.java.net/~stuefe/webrevs/8210068-Unsafe.allocateMemory-shall-not-round-up-user-size/webrev.01/webrev/

In Unsafe_AllocateMemory0(), we align the size requested by the caller
up like this:

  sz = align_up(sz, HeapWordSize);
  void* x = os::malloc(sz, mtOther);

This rounding is unnecessary, since the additional space is not needed
by the caller. In addition, this rounding causes NMT to over-report
the numbers of mtOther - which consists mainly of nio DirectByteBuffer
related allocations - the more buffers and the smaller they are the
more inaccurate those numbers become.

--

This bug has been originally reported by Adam Farley, see fragments of
a scattered discussion on core-libs:

http://mail.openjdk.java.net/pipermail/core-libs-dev/2018-July/054412.html
http://mail.openjdk.java.net/pipermail/core-libs-dev/2018-August/054797.html

--

The patch ran through jdk-submit. I will put it through our
SAP-internal tests to check whether removal of that alignment space
could possibly trigger any hidden overwrite bugs.

Thanks, Thomas