RFR: 8273872: ZGC: Explicitly use 2M large pages [v4]

[Per Liden]

On Thu, 16 Sep 2021 13:48:44 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> I don't think we should be passing `MAP_*` flags to `memfd_create()` as there's no guarantee that `MAP_HUGE_2MB` and `MFD_HUGE_2MB` are the same values. It's more of a lucky/convenient implementation detail that they happen to be the same.
>
> The manpages explicitly mention that the calculation is the same:
>>       MFD_HUGE_2MB, MFD_HUGE_1GB, ...
>>            Used in conjunction with MFD_HUGETLB to select alternative
>>            hugetlb page sizes (respectively, 2 MB, 1 GB, ...)  on
>>            systems that support multiple hugetlb page sizes.
>>            Definitions for known huge page sizes are included in the
>>            header file <linux/memfd.h>.
>>
>>            For details on encoding huge page sizes not included in
>>            the header file, see the discussion of the similarly named
>>            constants in mmap(2).
> 
> I.e. this is specified that way.

Ok. I think I'll stick with an explicit `MFD_HUGE_2MB` since that's always what we want to use.

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PR: https://git.openjdk.java.net/jdk/pull/5541