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

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Date:   Mon, 4 Jul 2022 19:19:09 +0300
From:   Sagi Grimberg <sagi@...mberg.me>
To:     Gabriel Krisman Bertazi <krisman@...labora.com>
Cc:     Ming Lei <ming.lei@...hat.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>
Subject: Re: [PATCH V3 1/1] ublk: add io_uring based userspace block driver


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