Btrfs: fix return value for __tree_mod_log_oldest_root

In __tree_mod_log_oldest_root() we must return the found operation even if
it's not a ROOT_REPLACE operation. Otherwise, the caller assumes that there
are no operations to be rewinded and returns immediately.

The code in the caller is modified to improve readability.
Signed-off-by: Jan Schmidt <list.btrfs@jan-o-sch.net>

fs/btrfs/ctree.c

@@ -1023,6 +1023,10 @@ __tree_mod_log_oldest_root(struct btrfs_fs_info *fs_info,
 		looped = 1;
 	}
 
+	/* if there's no old root to return, return what we found instead */
+	if (!found)
+		found = tm;
+
 	return found;
 }
 
@@ -1155,18 +1159,24 @@ get_old_root(struct btrfs_root *root, u64 time_seq)
 {
 	struct tree_mod_elem *tm;
 	struct extent_buffer *eb;
-	struct tree_mod_root *old_root;
+	struct tree_mod_root *old_root = NULL;
 	u64 old_generation;
+	u64 logical;
 
 	eb = btrfs_read_lock_root_node(root);
 	tm = __tree_mod_log_oldest_root(root->fs_info, root, time_seq);
 	if (!tm)
 		return root->node;
 
+	if (tm->op == MOD_LOG_ROOT_REPLACE) {
 		old_root = &tm->old_root;
 		old_generation = tm->generation;
+		logical = old_root->logical;
+	} else {
+		logical = root->node->start;
+	}
 
-	tm = tree_mod_log_search(root->fs_info, old_root->logical, time_seq);
+	tm = tree_mod_log_search(root->fs_info, logical, time_seq);
 	/*
 	 * there was an item in the log when __tree_mod_log_oldest_root
 	 * returned. this one must not go away, because the time_seq passed to
@@ -1174,26 +1184,23 @@ get_old_root(struct btrfs_root *root, u64 time_seq)
 	 */
 	BUG_ON(!tm);
 
-	if (old_root->logical == root->node->start) {
-		/* there are logged operations for the current root */
-		eb = btrfs_clone_extent_buffer(root->node);
-	} else {
-		/* there's a root replace operation for the current root */
+	if (old_root)
 		eb = alloc_dummy_extent_buffer(tm->index << PAGE_CACHE_SHIFT,
 					       root->nodesize);
-	}
+	else
+		eb = btrfs_clone_extent_buffer(root->node);
 	btrfs_tree_read_unlock(root->node);
 	free_extent_buffer(root->node);
 	if (!eb)
 		return NULL;
 	btrfs_tree_read_lock(eb);
-	if (old_root->logical != root->node->start) {
+	if (old_root) {
 		btrfs_set_header_bytenr(eb, eb->start);
 		btrfs_set_header_backref_rev(eb, BTRFS_MIXED_BACKREF_REV);
 		btrfs_set_header_owner(eb, root->root_key.objectid);
-	}
 		btrfs_set_header_level(eb, old_root->level);
 		btrfs_set_header_generation(eb, old_generation);
+	}
 	__tree_mod_log_rewind(eb, time_seq, tm);
 	extent_buffer_get(eb);