_posixsubprocess already uses the Linux getdent64 syscall when available (though for different reasons: readdir is not safe in that context). http://hg.python.org/cpython/file/3f3cbfd52f94/Modules/_posixsubprocess.c#l227

Probing for procfs at configure time could be problematic. It is a virtual filesystem. It is entirely possible for a system to choose not to mount it. It might be reasonable to assume that it "might be present" only if the system had it mounted at compile time but a configure flag to override that might be desirable for some systems (not the Linux systems I usually deal with).

If we're going through all of these hoops for closerange: I'd love to see an API exposed in the os module to return a list of open fd's. It is an abstraction nobody should have to write for themselves.