[PATCH 2/7] btrfs: basic direct I/O read operation

[Goldwyn Rodrigues <rgoldwyn@suse.de>]

From: Goldwyn Rodrigues <rgoldwyn@suse.com>

We will be getting rid of calling blockdev_direct_io. So, we need
to make sure we perform btrfs_direct_read() to cover the reads.

Introduce iomap begin function which calls get_iomap() to convert
the position to iomap. Use iomap_dio_rw() to perform the direct
reads.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
---
 fs/btrfs/file.c | 65 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 64 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 435a502a3226..eede9dcbb4b6 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -16,6 +16,7 @@
 #include <linux/btrfs.h>
 #include <linux/uio.h>
 #include <linux/iversion.h>
+#include <linux/iomap.h>
 #include "ctree.h"
 #include "disk-io.h"
 #include "transaction.h"
@@ -3415,6 +3416,57 @@ static int find_desired_extent(struct inode *inode, loff_t *offset, int whence)
 	return ret;
 }
 
+/*
+ * get_iomap: Get the block map and fill the iomap structure
+ * @pos: file position
+ * @length: I/O length
+ * @iomap: The iomap structure to fill
+ */
+
+static int get_iomap(struct inode *inode, loff_t pos, loff_t length,
+		struct iomap *iomap)
+{
+	struct extent_map *em;
+	iomap->addr = IOMAP_NULL_ADDR;
+	em = btrfs_get_extent(BTRFS_I(inode), NULL, 0, pos, length, 0);
+	if (IS_ERR(em))
+		return PTR_ERR(em);
+	/* XXX Do we need to check for em->flags here? */
+	if (em->block_start == EXTENT_MAP_HOLE) {
+		iomap->type = IOMAP_HOLE;
+	} else {
+		iomap->addr = em->block_start;
+		iomap->type = IOMAP_MAPPED;
+	}
+	iomap->offset = em->start;
+	iomap->bdev = em->bdev;
+	iomap->length = em->len;
+	free_extent_map(em);
+	return 0;
+}
+
+static int btrfs_dio_iomap_begin(struct inode *inode, loff_t pos,
+		loff_t length, unsigned flags, struct iomap *iomap,
+		struct iomap *srcmap)
+{
+	return get_iomap(inode, pos, length, iomap);
+}
+
+static const struct iomap_ops btrfs_dio_iomap_ops = {
+	.iomap_begin            = btrfs_dio_iomap_begin,
+};
+
+static ssize_t btrfs_dio_iomap_read(struct kiocb *iocb, struct iov_iter *to)
+{
+	struct inode *inode = file_inode(iocb->ki_filp);
+	ssize_t ret;
+	inode_lock_shared(inode);
+	ret = iomap_dio_rw(iocb, to, &btrfs_dio_iomap_ops, NULL,
+			is_sync_kiocb(iocb));
+	inode_unlock_shared(inode);
+	return ret;
+}
+
 static loff_t btrfs_file_llseek(struct file *file, loff_t offset, int whence)
 {
 	struct inode *inode = file->f_mapping->host;
@@ -3452,9 +3504,20 @@ static int btrfs_file_open(struct inode *inode, struct file *filp)
 	return generic_file_open(inode, filp);
 }
 
+static ssize_t btrfs_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
+{
+	ssize_t ret = 0;
+	if (iocb->ki_flags & IOCB_DIRECT)
+		ret = btrfs_dio_iomap_read(iocb, to);
+	if (ret < 0)
+		return ret;
+
+	return generic_file_buffered_read(iocb, to, ret);
+}
+
 const struct file_operations btrfs_file_operations = {
 	.llseek		= btrfs_file_llseek,
-	.read_iter      = generic_file_read_iter,
+	.read_iter      = btrfs_file_read_iter,
 	.splice_read	= generic_file_splice_read,
 	.write_iter	= btrfs_file_write_iter,
 	.mmap		= btrfs_file_mmap,
-- 
2.16.4