Btrfs: fix tree mod log for root replacements at leaf level

For the tree mod log, we don't log any operations at leaf level. If the root
is at the leaf level (i.e. the tree consists only of the root), then
__tree_mod_log_oldest_root will find a ROOT_REPLACE operation in the log
(because we always log that one no matter which level), but no other
operations.

With this patch __tree_mod_log_oldest_root exits cleanly instead of
BUGging in this situation. get_old_root checks if its really a root at leaf
level in case we don't have any operations and WARNs if this assumption
breaks.
Signed-off-by: Jan Schmidt <list.btrfs@jan-o-sch.net>

fs/btrfs/ctree.c

@@ -1024,11 +1024,18 @@ __tree_mod_log_oldest_root(struct btrfs_fs_info *fs_info,
 		if (!looped && !tm)
 			return 0;
 		/*
-		 * we must have key remove operations in the log before the
-		 * replace operation.
+		 * if there are no tree operation for the oldest root, we simply
+		 * return it. this should only happen if that (old) root is at
+		 * level 0.
 		 */
-		BUG_ON(!tm);
+		if (!tm)
+			break;
 
+		/*
+		 * if there's an operation that's not a root replacement, we
+		 * found the oldest version of our root. normally, we'll find a
+		 * MOD_LOG_KEY_REMOVE_WHILE_FREEING operation here.
+		 */
 		if (tm->op != MOD_LOG_ROOT_REPLACE)
 			break;
 
@@ -1192,16 +1199,8 @@ get_old_root(struct btrfs_root *root, u64 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
-	 * us must be blocking its removal.
-	 */
-	BUG_ON(!tm);
-
 	if (old_root)
-		eb = alloc_dummy_extent_buffer(tm->index << PAGE_CACHE_SHIFT,
-					       root->nodesize);
+		eb = alloc_dummy_extent_buffer(logical, root->nodesize);
 	else
 		eb = btrfs_clone_extent_buffer(root->node);
 	btrfs_tree_read_unlock(root->node);
@@ -1216,7 +1215,10 @@ get_old_root(struct btrfs_root *root, u64 time_seq)
 		btrfs_set_header_level(eb, old_root->level);
 		btrfs_set_header_generation(eb, old_generation);
 	}
-	__tree_mod_log_rewind(eb, time_seq, tm);
+	if (tm)
+		__tree_mod_log_rewind(eb, time_seq, tm);
+	else
+		WARN_ON(btrfs_header_level(eb) != 0);
 	extent_buffer_get(eb);
 
 	return eb;