Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Tue, 5 Jul 2022 08:43:43 +0800
From:   Ming Lei <ming.lei@redhat.com>
To:     Sagi Grimberg <sagi@grimberg.me>
Cc:     Gabriel Krisman Bertazi <krisman@collabora.com>,
        Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
        Harris James R <james.r.harris@intel.com>,
        linux-kernel@vger.kernel.org, io-uring@vger.kernel.org,
        ZiyangZhang <ZiyangZhang@linux.alibaba.com>,
        Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>,
        Stefan Hajnoczi <stefanha@redhat.com>, ming.lei@redhat.com
Subject: Re: [PATCH V3 1/1] ublk: add io_uring based userspace block driver
Message-ID: <YsOJP9BaD0LUpsgg@T590>
References: <20220628160807.148853-1-ming.lei@redhat.com>
 <20220628160807.148853-2-ming.lei@redhat.com>
 <da861bbb-1506-7598-fa06-32201456967d@grimberg.me>
 <YsLeR1QWPmqfNAQY@T590>
 <8cf1aef0-ea5b-a3df-266d-ae67674c96ae@grimberg.me>
 <87a69oamap.fsf@collabora.com>
 <c2053491-abb6-dc75-923d-bfea81431afa@grimberg.me>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <c2053491-abb6-dc75-923d-bfea81431afa@grimberg.me>
Precedence: bulk

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