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;
Message-ID: <49039d2b-e4bc-62a1-7b4a-a78001a2f430@veeam.com>
Date:   Thu, 7 Jul 2022 11:16:42 +0200
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.11.0
Subject: Re: [PATCH 15/20] block, blksnap: snapshot image block device
Content-Language: en-US
To:     Christoph Hellwig <hch@infradead.org>
CC:     "axboe@kernel.dk" <axboe@kernel.dk>,
        "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
References: <1655135593-1900-1-git-send-email-sergei.shtepa@veeam.com>
 <1655135593-1900-16-git-send-email-sergei.shtepa@veeam.com>
 <YsWKhjDV18DeyEZ8@infradead.org>
From:   Sergei Shtepa <sergei.shtepa@veeam.com>
In-Reply-To: <YsWKhjDV18DeyEZ8@infradead.org>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 7bit
Precedence: bulk


> 
>> +static int snapimage_init_request(struct blk_mq_tag_set *set,
>> +				  struct request *rq, unsigned int hctx_idx,
>> +				  unsigned int numa_node)
>> +{
>> +	struct snapimage_cmd *cmd = blk_mq_rq_to_pdu(rq);
>> +
>> +	kthread_init_work(&cmd->work, snapimage_queue_work);
>> +	return 0;
>> +}
> I'm a little confused on why this both has a blk-mq interface and
> the new bio filter interface.  Whow is going to submit I/O to this
> blk-mq interface?
> 

Sorry, it seems to me that I didn't understand this comment.

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.

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.

>> +	//.open = snapimage_open,
>> +	//.ioctl = snapimage_ioctl,
>> +	//.release = snapimage_close,
> Please don't leave commented out code around.
>