Re: [PATCH v7 0/8] add support for direct I/O with fscrypt using blk-crypto

[Darrick J. Wong]

On Tue, Nov 17, 2020 at 12:15:26PM -0500, Theodore Y. Ts'o wrote:
> 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.

In Ye Olde days there was XFS_IOC_DIOINFO to communicate all that (xfs
hardcodes 512b file offset alignment), but in this modern era perhaps
it's time to shovel that into statx...

--D

> 
> 						- Ted