RFR: 8262291: Refactor reserve_memory_special_huge_tlbfs [v3]

[Stefan Johansson]

> Please review this refactoring of the hugetlbfs reservation code.
> 
> **Summary**
> In recent adventures in this area of the code I noticed a strange condition in `reserve_memory_special_huge_tlbfs` where we take the "mixed-mapping" route even if the size doesn't require any small pages to be used:
>   if (is_aligned(bytes, os::large_page_size()) && alignment <= os::large_page_size()) {
>     return reserve_memory_special_huge_tlbfs_only(bytes, req_addr, exec);
>   } else {
>     return reserve_memory_special_huge_tlbfs_mixed(bytes, alignment, req_addr, exec);
>   }
> 
> The second condition here is needed because if the alignment is larger than the large page size, we needed to enforce this and can't just trust `mmap` to give us a properly aligned address. Doing this by using the mixed-function feels a bit weird and looking a bit more at this I found a way to refactor this function to avoid having the two helpers. 
> 
> Instead of only having the mixed path honor the passed down alignment, make sure that is always done. This will also have the side-effect that all large pages in a "mixed"-mapping will be at the start and then we will have a tail of small pages. This actually also ensures that we will use large pages for a mixed mapping, in the past there was a corner case where we could end up with just a head and tail of small pages and no large page in between (if the mapping was smaller than 2 large pages and there was no alignment constraint).
> 
> **Testing**
> Mach5 tier1-3 and a lot of local testing with different large page configurations.

Stefan Johansson has updated the pull request incrementally with one additional commit since the last revision:

  Marcus review.
  
  Updated comments.

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3073/files
  - new: https://git.openjdk.java.net/jdk/pull/3073/files/38a13144..2bf1d5a5

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3073&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3073&range=01-02

  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3073.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3073/head:pull/3073

PR: https://git.openjdk.java.net/jdk/pull/3073