RFR: 8261230: GC tracing of page sizes are wrong in a few places

[Albert Mingkun Yang]

On Tue, 9 Feb 2021 19:42:15 GMT, Stefan Johansson <sjohanss at openjdk.org> wrote:

> The usage of `os::trace_page_sizes()` and friends are wrongly assuming that we always get the page size requested and needs to be updated. This is done by using the helper `ReservedSpace::actual_reserved_page_size()` instead of blindly trusting we get what we ask for. I have plans for the future to get rid of this helper and instead record the page size used in the `ReservedSpace`, but for now the helper is good enough. 
> 
> In G1 we used the helper but switched the order of the page size and the alignment parameter, which in turn helped the test to pass since the alignment will match the page size we expect in the test. The test had to be improved to recognize mapping failures.

In the test, there's some string matching to detect if large page is properly set up. I think it's best to include an excerpt of the log showing both the success and failure modes in the comments. This way even readers who are not intimately familiar with the gc-logs output could still feel fairly confident that the output parsing part is indeed correct.

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Changes requested by ayang (Author).

PR: https://git.openjdk.java.net/jdk/pull/2486