Re: [PATCH 4/5] io_uring: add support for dma pre-mapping

[Keith Busch <kbusch@kernel.org>]

On Wed, Jul 27, 2022 at 03:04:56PM +0100, Al Viro wrote:
> 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...

Sorry if I'm misunderstanding your concern here.

The API is a file descriptor + index range of registered buffers (which is a
pre-existing io_uring API). The file descriptor can come from opening either a
raw block device (ex: /dev/nvme0n1), or any regular file on a mounted
filesystem using nvme as a backing store.

You don't need to know about specific block numbers. You can use the result
with any offset in the underlying block device.

This also isn't necessarily tied to nvme-pci; that's just the only low-level
driver I've enabled in this series, but others may come later.