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

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Date:   Thu, 7 Jul 2022 10:24:36 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Sergei Shtepa <sergei.shtepa@...am.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        "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.