btrfs: Factor out read portion of btrfs_get_blocks_direct

Currently this function handles both the READ and WRITE dio cases. This
is facilitated by a bunch of 'if' statements, a goto short-circuit
statement and a very perverse aliasing of "!created"(READ) case
by setting lockstart = lockend and checking for lockstart < lockend for
detecting the write. Let's simplify this mess by extracting the
READ-only code into a separate __btrfs_get_block_direct_read function.
This is only the first step, the next one will be to factor out the
write side as well. The end goal will be to have the common locking/
unlocking code in btrfs_get_blocks_direct and then it will call either
the read|write subvariants. No functional changes.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -7540,6 +7540,27 @@ static struct extent_map *create_io_em(struct inode *inode, u64 start, u64 len,
 	return em;
 }
 
+
+static int btrfs_get_blocks_direct_read(struct extent_map *em,
+					struct buffer_head *bh_result,
+					struct inode *inode,
+					u64 start, u64 len)
+{
+	if (em->block_start == EXTENT_MAP_HOLE ||
+			test_bit(EXTENT_FLAG_PREALLOC, &em->flags))
+		return -ENOENT;
+
+	len = min(len, em->len - (start - em->start));
+
+	bh_result->b_blocknr = (em->block_start + (start - em->start)) >>
+		inode->i_blkbits;
+	bh_result->b_size = len;
+	bh_result->b_bdev = em->bdev;
+	set_buffer_mapped(bh_result);
+
+	return 0;
+}
+
 static int btrfs_get_blocks_direct(struct inode *inode, sector_t iblock,
 				   struct buffer_head *bh_result, int create)
 {
@@ -7608,11 +7629,29 @@ static int btrfs_get_blocks_direct(struct inode *inode, sector_t iblock,
 		goto unlock_err;
 	}
 
-	/* Just a good old fashioned hole, return */
-	if (!create && (em->block_start == EXTENT_MAP_HOLE ||
-			test_bit(EXTENT_FLAG_PREALLOC, &em->flags))) {
+	if (!create) {
+		ret = btrfs_get_blocks_direct_read(em, bh_result, inode,
+						   start, len);
+		/* Can be negative only if we read from a hole */
+		if (ret < 0) {
+			ret = 0;
+			free_extent_map(em);
+			goto unlock_err;
+		}
+		/*
+		 * We need to unlock only the end area that we aren't using.
+		 * The rest is going to be unlocked by the endio routine.
+		 */
+		lockstart = start + bh_result->b_size;
+		if (lockstart < lockend) {
+			clear_extent_bit(&BTRFS_I(inode)->io_tree, lockstart,
+					 lockend, unlock_bits, 1, 0,
+					 &cached_state);
+		} else {
+			free_extent_state(cached_state);
+		}
 		free_extent_map(em);
-		goto unlock_err;
+		return 0;
 	}
 
 	/*
@@ -7624,12 +7663,6 @@ static int btrfs_get_blocks_direct(struct inode *inode, sector_t iblock,
 	 * just use the extent.
 	 *
 	 */
-	if (!create) {
-		len = min(len, em->len - (start - em->start));
-		lockstart = start + len;
-		goto unlock;
-	}
-
 	if (test_bit(EXTENT_FLAG_PREALLOC, &em->flags) ||
 	    ((BTRFS_I(inode)->flags & BTRFS_INODE_NODATACOW) &&
 	     em->block_start != EXTENT_MAP_HOLE)) {
@@ -7716,10 +7749,7 @@ unlock:
 		clear_extent_bit(&BTRFS_I(inode)->io_tree, lockstart,
 				 lockend, unlock_bits, 1, 0,
 				 &cached_state);
-	} else {
-		free_extent_state(cached_state);
 	}
-
 	free_extent_map(em);
 
 	return 0;