RFR: 8319795: Static huge pages are not used for CodeCache

[Thomas Stuefe]

On Thu, 9 Nov 2023 16:20:35 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> This is a fix of a regression introduced by [JDK-8261894](https://bugs.openjdk.org/browse/JDK-8261894).
>> After JDK-8261894, `os::can_execute_large_page_memory()` returns `true` only if `UseTransparentHugePages` is true. As CodeCache uses `os::can_execute_large_page_memory()` when it detects a page size, CodeCache cannot use static huge pages (`UseTransparentHugePages` is `false`) anymore after the change.
>> Before JDK-8261894, `os::can_execute_large_page_memory()` returned `true`  when either `UseTransparentHugePages` or `UseHugeTLBFS` was true.
>> 
>> After JDK-8261894, `XX:+UseLargePages XX:-UseTransparentHugePages` means to use static huge pages: aka `UseHugeTLBFS` is `true`. If `UseLargePages` is not set to `true` via the option, it will be set to `true` if `UseTransparentHugePages` is `true`.
>> 
>> `os::can_execute_large_page_memory()` is modified to return `UseLargePages`. A regression gtest  is added.
>> 
>> Tested fastdebug and release builds:
>> - [x] tier1
>> - [x] gtest
>> - [x] test/hotspot/jtreg/gtest/LargePageGtests.java
>
> We should have a regression test for this...

> @tstuefe Yes, removing the function makes sense to me. Without the function another regression test is needed because the one in this PR will be removed. I'll add the test.

Cool. BTW the test does not have to be complicated. 

Just start a VM with `java -Xmx64m -Xlog:pagesize* -XX:+UseLargePages -version` and scan the output.

If the output shows `UseLargePages=1`, the VM uses large pages, which means the test system has large pages. In that case, we also expect something like this:

`CodeHeap .. blabla .. page_size=2M`

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PR Comment: https://git.openjdk.org/jdk/pull/16582#issuecomment-1804284774