Btrfs: only process as many file extents as there are refs

The backref walking code will search down to the key it is looking for and then
proceed to walk _all_ of the extents on the file until it hits the end.  This is
suboptimal with large files, we only need to look for as many extents as we have
references for that inode.  I have a testcase that creates a randomly written 4
gig file and before this patch it took 6min 30sec to do the initial send, with
this patch it takes 2min 30sec to do the intial send.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/backref.c

@@ -209,18 +209,19 @@ static int __add_prelim_ref(struct list_head *head, u64 root_id,
 }
 
 static int add_all_parents(struct btrfs_root *root, struct btrfs_path *path,
-				struct ulist *parents, int level,
-				struct btrfs_key *key_for_search, u64 time_seq,
-				u64 wanted_disk_byte,
-				const u64 *extent_item_pos)
+			   struct ulist *parents, struct __prelim_ref *ref,
+			   int level, u64 time_seq, const u64 *extent_item_pos)
 {
 	int ret = 0;
 	int slot;
 	struct extent_buffer *eb;
 	struct btrfs_key key;
+	struct btrfs_key *key_for_search = &ref->key_for_search;
 	struct btrfs_file_extent_item *fi;
 	struct extent_inode_elem *eie = NULL, *old = NULL;
 	u64 disk_byte;
+	u64 wanted_disk_byte = ref->wanted_disk_byte;
+	u64 count = 0;
 
 	if (level != 0) {
 		eb = path->nodes[level];
@@ -238,7 +239,7 @@ static int add_all_parents(struct btrfs_root *root, struct btrfs_path *path,
 	if (path->slots[0] >= btrfs_header_nritems(path->nodes[0]))
 		ret = btrfs_next_old_leaf(root, path, time_seq);
 
-	while (!ret) {
+	while (!ret && count < ref->count) {
 		eb = path->nodes[0];
 		slot = path->slots[0];
 
@@ -254,6 +255,7 @@ static int add_all_parents(struct btrfs_root *root, struct btrfs_path *path,
 		if (disk_byte == wanted_disk_byte) {
 			eie = NULL;
 			old = NULL;
+			count++;
 			if (extent_item_pos) {
 				ret = check_extent_in_eb(&key, eb, fi,
 						*extent_item_pos,
@@ -334,9 +336,8 @@ static int __resolve_indirect_ref(struct btrfs_fs_info *fs_info,
 		eb = path->nodes[level];
 	}
 
-	ret = add_all_parents(root, path, parents, level, &ref->key_for_search,
-				time_seq, ref->wanted_disk_byte,
-				extent_item_pos);
+	ret = add_all_parents(root, path, parents, ref, level, time_seq,
+			      extent_item_pos);
 out:
 	path->lowest_level = 0;
 	btrfs_release_path(path);