linux-kernel - Re: [RFC PATCH 4/4] ublk_drv: support splice based read/write zero copy

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Message-ID: <712cd802-f3bb-9840-e334-385cd42325f2@fastmail.fm>
Date:   Thu, 3 Nov 2022 23:28:29 +0100
From:   Bernd Schubert <bernd.schubert@...tmail.fm>
To:     Ming Lei <ming.lei@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        io-uring@...r.kernel.org
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        Miklos Szeredi <mszeredi@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        ZiyangZhang <ZiyangZhang@...ux.alibaba.com>
Subject: Re: [RFC PATCH 4/4] ublk_drv: support splice based read/write zero
 copy

On 11/3/22 09:50, Ming Lei wrote:
> Pass ublk block IO request pages to kernel backend IO handling code via
> pipe, and request page copy can be avoided. So far, the existed
> pipe/splice mechanism works for handling write request only.
> 
> The initial idea of using splice for zero copy is from Miklos and Stefan.
> 
> Read request's zero copy requires pipe's change to allow one read end to
> produce buffers for another read end to consume. The added SPLICE_F_READ_TO_READ
> flag is for supporting this feature.
> 
> READ is handled by sending IORING_OP_SPLICE with SPLICE_F_DIRECT |
> SPLICE_F_READ_TO_READ. WRITE is handled by sending IORING_OP_SPLICE with
> SPLICE_F_DIRECT. Kernel internal pipe is used for simplifying userspace,
> meantime potential info leak could be avoided.

Sorry to ask, do you have an ublk branch that gives an example how to 
use this?

I still have several things to fix in my branches, but I got basic fuse 
uring with copies working. Adding back splice would be next after 
posting rfc patches. My initial assumption was that I needed to 
duplicate everything splice does into the fuse .uring_cmd handler - 
obviously there is a better way with your patches.

This week I have a few days off, by end of next week or the week after I 
might have patches in an rfc state (one thing I'm going to ask about is 
how do I know what is the next CQE in the kernel handler - ublk does 
this with tags through mq, but I don't understand yet where the tag is 
increased and what the relation between tag and right CQE order is).

This got modeled a bit after ublk, but then diverged a bit.

https://github.com/aakefbs/linux/tree/fuse-uring
https://github.com/aakefbs/libfuse/tree/uring

(Again, the branches are not ready by any means for review yet).

Thanks,
Bernd