[PATCH v3] Btrfs: optimize btrfs_lookup_extent_info()

[Filipe David Borba Manana <fdmanana@gmail.com>]

If we're looking for a metadata item in the tree and the
search fails with return value of 1, and the slot doesn't
point to the first item in the leaf, check if the previous
item in the leaf corresponds to an extent item for the same
object id - if it does, then don't do another tree search
to get it.

This optimization is already done by btrfs-progs.

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
---

V2: updated commit message to mention this is already done in
    btrfs-progs.
V3: moved version information out of the commit message, as
    David Sterba notified me this is the correct way of
    adding patch version information.

 fs/btrfs/extent-tree.c |   21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 0236de7..4b19c88 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -771,10 +771,23 @@ again:
 		goto out_free;
 
 	if (ret > 0 && metadata && key.type == BTRFS_METADATA_ITEM_KEY) {
-		key.type = BTRFS_EXTENT_ITEM_KEY;
-		key.offset = root->leafsize;
-		btrfs_release_path(path);
-		goto again;
+		metadata = 0;
+		if (path->slots[0]) {
+			path->slots[0]--;
+			btrfs_item_key_to_cpu(path->nodes[0], &key,
+					      path->slots[0]);
+			if (key.objectid == bytenr &&
+			    key.type == BTRFS_EXTENT_ITEM_KEY &&
+			    key.offset == root->leafsize)
+				ret = 0;
+		}
+		if (ret) {
+			key.objectid = bytenr;
+			key.type = BTRFS_EXTENT_ITEM_KEY;
+			key.offset = root->leafsize;
+			btrfs_release_path(path);
+			goto again;
+		}
 	}
 
 	if (ret == 0) {
-- 
1.7.9.5