Hang / zombie process from Xarray page-fault conversion (bisected)

Hi Willy, We're seeing a case where RocksDB hangs and becomes defunct when trying to kill the process. v4.19 succeeds and v4.20 fails. Robert was able to bisect this to commit b15cd800682f "dax: Convert page fault handlers to XArray". I see some direct usage of xa_index and wonder if there are some more pmd fixups to do? Other thoughts?