According to this article
http://lwn.net/Articles/423584/
The current patch only works with anonymous pages; work integration of huge pages with the page cache has not yet been completed. It also handles only one huge page size (2 MB). However, some useful performance improvements. Mel Gorman conducted several tests showing improvements of up to 10% or so in some situations. The overall results were not as good as you would get with hugetlbfs, but THP is much more likely to be used.
So, it seems that THP support is implemented for merging / splitting 4k → 2M / 4M.
In addition, in the sources of the kernel, I saw that there is a split_huge_page_pmd function and how PMD undergoes separation and PMD controls at the top of PTE's , which has a fixed size of 4k, I assumed that the size of PMD (2M / 4M) is the maximum transparent size of a huge pages.