Btw, I just had a chat with Dan last week on this. And he pointed out that while this series deals with the read/write path issues of reflink on DAX it doesn't deal with the mmap side issue that page->mapping and page->index can point back to exactly one file.
I think we want a few xfstests that reflink a file and then use the different links using mmap, as that should blow up pretty reliably.
