RFR: 8262291: Refactor reserve_memory_special_huge_tlbfs

[Stefan Johansson]

On Thu, 18 Mar 2021 14:00:00 GMT, Stefan Johansson <sjohanss at openjdk.org> wrote:

> 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.

Thanks for reviewing Ivan, I will wait to update the PR until we come up with a function name that is better than the current.

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