Re: [PATCH RFC v2 00/19] fuse: fuse-over-io-uring

[Bernd Schubert <bernd.schubert@fastmail.fm>]

On 5/30/24 17:36, Kent Overstreet wrote:
> On Wed, May 29, 2024 at 08:00:35PM +0200, Bernd Schubert wrote:
>> From: Bernd Schubert <bschubert@ddn.com>
>>
>> This adds support for uring communication between kernel and
>> userspace daemon using opcode the IORING_OP_URING_CMD. The basic
>> appraoch was taken from ublk.  The patches are in RFC state,
>> some major changes are still to be expected.
>>
>> Motivation for these patches is all to increase fuse performance.
>> In fuse-over-io-uring requests avoid core switching (application
>> on core X, processing of fuse server on random core Y) and use
>> shared memory between kernel and userspace to transfer data.
>> Similar approaches have been taken by ZUFS and FUSE2, though
>> not over io-uring, but through ioctl IOs
> 
> What specifically is it about io-uring that's helpful here? Besides the
> ringbuffer?
> 
> So the original mess was that because we didn't have a generic
> ringbuffer, we had aio, tracing, and god knows what else all
> implementing their own special purpose ringbuffers (all with weird
> quirks of debatable or no usefulness).
> 
> It seems to me that what fuse (and a lot of other things want) is just a
> clean simple easy to use generic ringbuffer for sending what-have-you
> back and forth between the kernel and userspace - in this case RPCs from
> the kernel to userspace.
> 
> But instead, the solution seems to be just toss everything into a new
> giant subsystem?


Hmm, initially I had thought about writing my own ring buffer, but then 
io-uring got IORING_OP_URING_CMD, which seems to have exactly what we
need? From interface point of view, io-uring seems easy to use here, 
has everything we need and kind of the same thing is used for ublk - 
what speaks against io-uring? And what other suggestion do you have?

I guess the same concern would also apply to ublk_drv. 

Well, decoupling from io-uring might help to get for zero-copy, as there
doesn't seem to be an agreement with Mings approaches (sorry I'm only
silently following for now).

From our side, a customer has pointed out security concerns for io-uring. 
My thinking so far was to implemented the required io-uring pieces into 
an module and access it with ioctls... Which would also allow to
backport it to RHEL8/RHEL9.


Thanks,
Bernd