From: Goldwyn Rodrigues <rgoldwyn@suse.com>

btrfs_read_iomap_begin() fetches the extent on the file position passed
and converts the resultant extent map to iomap. The iomap code uses the
sector offset from iomap structure to create the bios.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
---
 fs/btrfs/extent_io.c | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 7f40c2b8bfb8..6ef2fa802c30 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -962,6 +962,34 @@ static struct extent_map *__get_extent_map(struct inode *inode,
 
 	return em;
 }
+
+static int btrfs_read_iomap_begin(struct inode *inode, loff_t pos,
+		loff_t length, unsigned int flags, struct iomap *iomap,
+		struct iomap *srcmap)
+{
+	struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
+	struct extent_map *em;
+	struct extent_state *cached_state = NULL;
+	u64 start = round_down(pos, fs_info->sectorsize);
+	u64 end = round_up(pos + length, fs_info->sectorsize) - 1;
+
+	btrfs_lock_and_flush_ordered_range(BTRFS_I(inode), start, end, &cached_state);
+	em = btrfs_get_extent(BTRFS_I(inode), NULL, start, end - start + 1);
+	unlock_extent(&BTRFS_I(inode)->io_tree, start, end, &cached_state);
+	if (IS_ERR(em))
+		return PTR_ERR(em);
+
+	btrfs_em_to_iomap(inode, em, iomap, start, false);
+	free_extent_map(em);
+
+	return 0;
+}
+
+static const struct iomap_ops btrfs_buffered_read_iomap_ops = {
+	.iomap_begin = btrfs_read_iomap_begin,
+};
+
+
 /*
  * basic readpage implementation.  Locked extent state structs are inserted
  * into the tree that are removed when the IO is done (by the end_io
-- 
2.46.1