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

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