RFR (xs): 8211735: Wrong heap mapper can be selected with UseLargePages on G1

[Kim Barrett]

> On Nov 7, 2018, at 4:22 PM, sangheon.kim at oracle.com wrote:
> 
> Hi all,
> 
> Can I have reviews for this tiny patch fixes wrong heap mapper selection with UseLargePages on G1?
> 
> When we enable UseLargePages but failed allocating with large page, G1 is not correctly reflecting it.
> This situation only happens when G1HeapRegionSize is less than or equal to large page size.
> e.g. G1HeapRegionsSize=1MB, default page size=4KB, large page size=2MB. G1 selects G1RegionsSmallerThanCommitSizeMapper (G1HeapRegionSize < large page size, at G1RegionToSpaceMapper::create_mapper()) but actually we are using default page(failed allocating with large page) so G1RegionsLargerThanCommitSizeMapper should be selected. Eventually G1 commits memory with not optimal way(it works) and pages will not touched properly when AlwaysPreTouch option is enabled.
> 
> CR: https://bugs.openjdk.java.net/browse/JDK-8211735
> Webrev: http://cr.openjdk.java.net/~sangheki/8211735/webrev.0
> Testing: hs-tier1, hs-tier2, hs-tier3

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src/hotspot/share/gc/g1/g1CollectedHeap.cpp
1663     if (os::can_commit_large_page_memory() ||
1664         // If we want large page and current os doesn't support committing large page memory,
1665         // we manage differently at ReservedSpace and it is called 'special'.
1666         // If we fail to set 'special', we reserved memory without large page.
1667         (!os::can_commit_large_page_memory() && heap_rs.special())) {

That's really confusing.
Since "x || (!x && y) => x || y", this should be

  if (os::can_commit_large_page_memory() || heap_rs.special()) {

and update the comment accordingly.

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