linux-kernel - Re: [PATCH 15/20] block, blksnap: snapshot image block device

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Date:   Fri, 8 Jul 2022 09:58:25 +0200
From:   Sergei Shtepa <sergei.shtepa@...am.com>
To:     Christoph Hellwig <hch@...radead.org>
CC:     "axboe@...nel.dk" <axboe@...nel.dk>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 15/20] block, blksnap: snapshot image block device


> On Thu, Jul 07, 2022 at 11:16:42AM +0200, Sergei Shtepa wrote:
>> The module creates a block device for each snapshot image.
>> To make a backup of a block device, the backup tool reads snapshot image.
>> This snapshot image block device allows to mount a file system on it
>> and perform the necessary preparation. If not for this requirement,
>> it would be possible to implement reading via an additional ioctl.
>> But that wouldn't be a good design, I think.
> Ok, got it.  It was just me who was confused.
> 
>> Perhaps I have implemented this block device incorrectly?
>> Processing requests of the snapshot image block device is started
>> in the function snapimage_queue_rq(). And ends in the 
>> snapimage_queue_work() in another kernel thread. Therefore, when
>> the request is initialized in snapimage_init_request(), a kernel worker
>> is prepared.
> I don't think it is wrong, but there is some potential for optimization.
> 
> Is there a reson this is implemented as a blk-mq driver vs a bio
> based driver that just implements ->submit_bio?  The latter has
> the advantage that you are always called in user context, and don't
> need the extra workqueue offload. 
> 
> The block layer also generally assumes that blk-mq drivers don't
> call submit_bio_noacct underneath, so if we can't I'd much
> prefer the bio based driver approach here.
> 

There was a goal to use the kernel innovations whenever possible.
Of course, it makes sense to return to bio based if it allows to
achieve better performance. This is not a matter of principle.