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Date:   Wed, 6 Jul 2022 06:13:42 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Sergei Shtepa <sergei.shtepa@veeam.com>
Cc:     axboe@kernel.dk, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 15/20] block, blksnap: snapshot image block device
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> +#include <linux/cdrom.h>

What do you need this for?

> +static int new_minor(int *minor, void *ptr)
> +{
> +	int ret;
> +
> +	idr_preload(GFP_KERNEL);
> +	spin_lock(&_minor_lock);
> +
> +	ret = idr_alloc(&_minor_idr, ptr, 0, 1 << MINORBITS, GFP_NOWAIT);
> +
> +	spin_unlock(&_minor_lock);
> +	idr_preload_end();
> +
> +	if (ret < 0)
> +		return ret;
> +
> +	*minor = ret;
> +	return 0;
> +}

alloc_disk not automatically allocates minors for you if you don't
set the ->major and ->first_minor fields, which could simply this
a lot.

> +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?

> +	//.open = snapimage_open,
> +	//.ioctl = snapimage_ioctl,
> +	//.release = snapimage_close,

Please don't leave commented out code around.