What is the expected use case for Direct I/O using fscrypt?  This
isn't a problem which is unique to fscrypt, but one of the really
unfortunate aspects of the DIO interface is the silent fallback to
buffered I/O.  We've lived with this because DIO goes back decades,
and the original use case was to keep enterprise databases happy, and
the rules around what is necessary for DIO to work was relatively well
understood.

But with fscrypt, there's going to be some additional requirements
(e.g., using inline crypto) required or else DIO silently fall back to
buffered I/O for encrypted files.  Depending on the intended use case
of DIO with fscrypt, this caveat might or might not be unfortunately
surprising for applications.

I wonder if we should have some kind of interface so we can more
explicitly allow applications to query exactly what the requirements
might be for a particular file vis-a-vis Direct I/O.  What are the
memory alignment requirements, what are the file offset alignment
requirements, what are the write size requirements, for a particular
file.

						- Ted