Re: [LSF/MM TOPIC] Direct block mapping through fs for device

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Fri, Apr 26, 2019 at 05:45:53AM -0700, Christoph Hellwig wrote:
> On Fri, Apr 26, 2019 at 04:28:16PM +1000, Dave Chinner wrote:
> > i.e. go look at how xfs_pnfs.c works to hand out block mappings to
> > remote pNFS clients so they can directly access the underlying
> > storage. Basically, anyone wanting to map blocks needs a file layout
> > lease and then to manage the filesystem state over that range via
> > these methods in the struct export_operations:
> > 
> >         int (*get_uuid)(struct super_block *sb, u8 *buf, u32 *len, u64 *offset);
> >         int (*map_blocks)(struct inode *inode, loff_t offset,
> >                           u64 len, struct iomap *iomap,
> >                           bool write, u32 *device_generation);
> >         int (*commit_blocks)(struct inode *inode, struct iomap *iomaps,
> >                              int nr_iomaps, struct iattr *iattr);
> 
> Nipick:  get_uuid isn't needed for the least itself, it just works
> around the fact that the original pNFS/block protocol is braindead.
> The pNFS/SCSI prototocol already switches to a device UUID, and other
> users that work locally shouldn't need it either.

Hmmm, this lease interface still doesn't support COW, right?

(Right, xfs_pnfs.c bails out with -ENXIO for reflink files)

It occurs to me that maybe we don't want Goldwyn's IOMAP_DAX_COW
approach (hide the read address in the iomap->inline_data pointer); we
just want two physical source addresses.  Then the dax code can turn
that into a memory pointer and file lessees can do sector accesses or
whatever they need to do to write the range before calling
->commit_blocks.

Oh, right, both of you commented about a dual iomap approach on the v2
btrfs dax support series.

/me goes back to drinking coffee.

--D