linux-kernel - Re: [PATCH V3 1/1] ublk: add io_uring based userspace block driver

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Date:   Tue, 5 Jul 2022 08:43:43 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Sagi Grimberg <sagi@...mberg.me>
Cc:     Gabriel Krisman Bertazi <krisman@...labora.com>,
        Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        Harris James R <james.r.harris@...el.com>,
        linux-kernel@...r.kernel.org, io-uring@...r.kernel.org,
        ZiyangZhang <ZiyangZhang@...ux.alibaba.com>,
        Xiaoguang Wang <xiaoguang.wang@...ux.alibaba.com>,
        Stefan Hajnoczi <stefanha@...hat.com>, ming.lei@...hat.com
Subject: Re: [PATCH V3 1/1] ublk: add io_uring based userspace block driver

On Mon, Jul 04, 2022 at 07:19:09PM +0300, Sagi Grimberg wrote:
> 
> > > > > > diff --git a/drivers/block/Kconfig b/drivers/block/Kconfig
> > > > > > index fdb81f2794cd..d218089cdbec 100644
> > > > > > --- a/drivers/block/Kconfig
> > > > > > +++ b/drivers/block/Kconfig
> > > > > > @@ -408,6 +408,12 @@ config BLK_DEV_RBD
> > > > > >     	  If unsure, say N.
> > > > > > +config BLK_DEV_UBLK
> > > > > > +	bool "Userspace block driver"
> > > > > 
> > > > > Really? why compile this to the kernel and not tristate as loadable
> > > > > module?
> > > > So far, this is only one reason: task_work_add() is required, which
> > > > isn't exported for modules.
> > > 
> > > So why not exporting it?
> > > Doesn't seem like a good justification to build it into the kernel.
> > 
> > Sagi,
> > 
> > If I understand correctly, the task_work_add function is quite a core
> > API that we probably want to avoid exposing directly to (out-of-tree)
> > modules?  I agree, though, it would be great to have this buildable as a
> > module for general use cases.  Would it make sense to have it exposed
> > through a thin built-in wrapper, specific to UBD, which is exported, and
> > therefore able to invoke that function?  Is it a reasonable approach?
> 
> All I'm saying is that either we should expose it (or an interface to
> it) if it has merit, or use something else (use a workqueue).

There isn't replacement for task_work_add().

If module has to be supported, we can add one command for running the
work function in the ubq context, that will add some cost.

> Having a block driver driver builtin is probably not the answer.

Not sure, there are at least two drivers which use the API.


Thanks,
Ming