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Date:   Wed, 27 Jul 2022 15:04:56 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Keith Busch <kbusch@kernel.org>
Cc:     Keith Busch <kbusch@fb.com>, 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
Subject: Re: [PATCH 4/5] io_uring: add support for dma pre-mapping
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On Wed, Jul 27, 2022 at 07:58:29AM -0600, Keith Busch wrote:
> On Wed, Jul 27, 2022 at 12:12:53AM +0100, Al Viro wrote:
> > On Tue, Jul 26, 2022 at 10:38:13AM -0700, Keith Busch wrote:
> > 
> > > +	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;
> > 
> > *blink*
> > 
> > Just what's the intended use of the second case here?
> 
> ??
> 
> The use case is same as the first's: dma map the user addresses to the backing
> storage. There's two cases here because getting the block_device for a regular
> filesystem file is different than a raw block device.

Excuse me, but "file on some filesystem + block number on underlying device"
makes no sense as an API...