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Date:   Wed, 27 Jul 2022 15:11:05 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Keith Busch <kbusch@fb.com>
Cc:     linux-nvme@lists.infradead.org, linux-block@vger.kernel.org,
        io-uring@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        axboe@kernel.dk, hch@lst.de, Keith Busch <kbusch@kernel.org>
Subject: Re: [PATCH 4/5] io_uring: add support for dma pre-mapping
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References: <20220726173814.2264573-1-kbusch@fb.com>
 <20220726173814.2264573-5-kbusch@fb.com>
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On Tue, Jul 26, 2022 at 10:38:13AM -0700, Keith Busch wrote:

> +	file = fget(map.fd);
> +	if (!file)
> +		return -EBADF;
> +
> +	if (S_ISBLK(file_inode(file)->i_mode))
> +		bdev = I_BDEV(file->f_mapping->host);
> +	else if (S_ISREG(file_inode(file)->i_mode))
> +		bdev = file->f_inode->i_sb->s_bdev;
> +	else
> +		return -EOPNOTSUPP;
> +
> +	for (i = map.buf_start; i < map.buf_end; i++) {
> +		struct io_mapped_ubuf *imu = ctx->user_bufs[i];
> +		void *tag;
> +
> +		if (imu->dma_tag) {
> +			ret = -EBUSY;
> +			goto err;
> +		}
> +
> +		tag = block_dma_map(bdev, imu->bvec, imu->nr_bvecs);
> +		if (IS_ERR(tag)) {
> +			ret = PTR_ERR(tag);
> +			goto err;
> +		}
> +
> +		imu->dma_tag = tag;
> +		imu->dma_file = file;
> +		imu->bdev = bdev;
> +	}
> +
> +	fput(file);

This, BTW, is completely insane - what happens if you follow that
with close(map.fd)?  A bunch of dangling struct file references?

I really don't understand what you are trying to do here.