btrfs: remove partial support for lowest level from btrfs_search_forward()

Commit 323ac95 ("Btrfs: don't read leaf blocks containing only
checksums during truncate") changed the condition from `level == 0` to
`level == path->lowest_level`, while its original purpose is just to do
some leaf node handling (calling btrfs_item_key_to_cpu()) and skip some
code that doesn't fit leaf nodes.

After changing the condition, the code path:

1. Also handles the non-leaf nodes when path->lowest_level is nonzero,
   which is wrong. However btrfs_search_forward() is never called with a
   nonzero path->lowest_level, which makes this bug not found before.

2. Makes the later if block with the same condition, which is originally
   used to handle non-leaf node (calling btrfs_node_key_to_cpu()) when
   lowest_level is not zero, dead code.

Since btrfs_search_forward() is never called for a path with a
lowest_level different from zero, just completely remove the partial
support for a non-zero lowest_level, simplifying a bit the code, and
assert that lowest_level is zero at the start of the function.

Suggested-by: Qu Wenruo <wqu@suse.com>
Fixes: 323ac95 ("Btrfs: don't read leaf blocks containing only checksums during truncate")
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Sun YangKai <sunk67188@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: Sun YangKai

fs/btrfs/ctree.c

@@ -4594,16 +4594,13 @@ int btrfs_del_items(struct btrfs_trans_handle *trans, struct btrfs_root *root,
 
 /*
  * A helper function to walk down the tree starting at min_key, and looking
- * for nodes or leaves that are have a minimum transaction id.
+ * for leaves that have a minimum transaction id.
  * This is used by the btree defrag code, and tree logging
  *
  * This does not cow, but it does stuff the starting key it finds back
  * into min_key, so you can call btrfs_search_slot with cow=1 on the
  * key and get a writable path.
  *
- * This honors path->lowest_level to prevent descent past a given level
- * of the tree.
- *
  * min_trans indicates the oldest transaction that you are interested
  * in walking through.  Any nodes or leaves older than min_trans are
  * skipped over (without reading them).
@@ -4624,6 +4621,7 @@ int btrfs_search_forward(struct btrfs_root *root, struct btrfs_key *min_key,
 	int keep_locks = path->keep_locks;
 
 	ASSERT(!path->nowait);
+	ASSERT(path->lowest_level == 0);
 	path->keep_locks = 1;
 again:
 	cur = btrfs_read_lock_root_node(root);
@@ -4645,8 +4643,8 @@ int btrfs_search_forward(struct btrfs_root *root, struct btrfs_key *min_key,
 			goto out;
 		}
 
-		/* at the lowest level, we're done, setup the path and exit */
-		if (level == path->lowest_level) {
+		/* At level 0 we're done, setup the path and exit. */
+		if (level == 0) {
 			if (slot >= nritems)
 				goto find_next_key;
 			ret = 0;
@@ -4687,12 +4685,6 @@ int btrfs_search_forward(struct btrfs_root *root, struct btrfs_key *min_key,
 				goto out;
 			}
 		}
-		if (level == path->lowest_level) {
-			ret = 0;
-			/* Save our key for returning back. */
-			btrfs_node_key_to_cpu(cur, min_key, slot);
-			goto out;
-		}
 		cur = btrfs_read_node_slot(cur, slot);
 		if (IS_ERR(cur)) {
 			ret = PTR_ERR(cur);
@@ -4708,7 +4700,7 @@ int btrfs_search_forward(struct btrfs_root *root, struct btrfs_key *min_key,
 out:
 	path->keep_locks = keep_locks;
 	if (ret == 0)
-		btrfs_unlock_up_safe(path, path->lowest_level + 1);
+		btrfs_unlock_up_safe(path, 1);
 	return ret;
 }