RFR: 8262291: Refactor reserve_memory_special_huge_tlbfs [v6]

[Thomas Stuefe]

On Mon, 12 Apr 2021 09:31:22 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.
>
> Stefan Johansson has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:
> 
>  - Merge branch 'master' into 8262291-one-special-alloc
>  - Thomas review.
>    
>    Changed commit_memory_special to return bool to signal if the request succeeded or not.
>  - Self review.
>    
>    Update helper name to better match commit_memory_special().
>  - Marcus review.
>    
>    Updated comments.
>  - Ivan review
>    
>    Renamed helper to commit_memory_special and updated the comments.
>  - 8262291: Refactor reserve_memory_special_huge_tlbfs

Looks good to me.

Thanks, Thomas

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