Btrfs: Find and remove dead roots the first time a root is loaded.

Dead roots are trees left over after a crash, and they were either in the
process of being removed or were waiting to be removed when the box crashed.
Before, a search of the entire tree of root pointers was done on mount
looking for dead roots.  Now, the search is done the first time we load
a root.

This makes mount faster when there are a large number of snapshots, and it
enables the block accounting code to properly update the block counts on
the latest root as old versions of the root are reaped after a crash.
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/ctree.h

@@ -1148,7 +1148,8 @@ int btrfs_update_root(struct btrfs_trans_handle *trans, struct btrfs_root
 		      *item);
 int btrfs_find_last_root(struct btrfs_root *root, u64 objectid, struct
 			 btrfs_root_item *item, struct btrfs_key *key);
-int btrfs_find_dead_roots(struct btrfs_root *root);
+int btrfs_find_dead_roots(struct btrfs_root *root, u64 objectid,
+			  struct btrfs_root *latest_root);
 /* dir-item.c */
 int btrfs_insert_dir_item(struct btrfs_trans_handle *trans, struct btrfs_root
 			  *root, const char *name, int name_len, u64 dir,

fs/btrfs/disk-io.c

@@ -426,6 +426,10 @@ struct btrfs_root *btrfs_read_fs_root(struct btrfs_fs_info *fs_info,
 		return ERR_PTR(ret);
 	}
 
+	ret = btrfs_find_dead_roots(fs_info->tree_root,
+				    root->root_key.objectid, root);
+	BUG_ON(ret);
+
 	return root;
 }
 
@@ -522,11 +526,6 @@ struct btrfs_root *open_ctree(struct super_block *sb)
 	btrfs_read_block_groups(extent_root);
 
 	fs_info->generation = btrfs_super_generation(disk_super) + 1;
-	ret = btrfs_find_dead_roots(tree_root);
-	if (ret) {
-		mutex_unlock(&fs_info->fs_mutex);
-		goto fail_tree_root;
-	}
 	mutex_unlock(&fs_info->fs_mutex);
 	return tree_root;
 

fs/btrfs/root-tree.c

@@ -93,7 +93,8 @@ int btrfs_insert_root(struct btrfs_trans_handle *trans, struct btrfs_root
 	return ret;
 }
 
-int btrfs_find_dead_roots(struct btrfs_root *root)
+int btrfs_find_dead_roots(struct btrfs_root *root, u64 objectid,
+			  struct btrfs_root *latest)
 {
 	struct btrfs_root *dead_root;
 	struct btrfs_item *item;
@@ -105,7 +106,7 @@ int btrfs_find_dead_roots(struct btrfs_root *root)
 	struct btrfs_leaf *leaf;
 	int slot;
 
-	key.objectid = 0;
+	key.objectid = objectid;
 	key.flags = 0;
 	btrfs_set_key_type(&key, BTRFS_ROOT_ITEM_KEY);
 	key.offset = 0;
@@ -131,15 +132,24 @@ int btrfs_find_dead_roots(struct btrfs_root *root)
 		btrfs_disk_key_to_cpu(&key, &item->key);
 		if (btrfs_key_type(&key) != BTRFS_ROOT_ITEM_KEY)
 			goto next;
+
+		if (key.objectid < objectid)
+			goto next;
+
+		if (key.objectid > objectid)
+			break;
+
 		ri = btrfs_item_ptr(leaf, slot, struct btrfs_root_item);
 		if (btrfs_root_refs(ri) != 0)
 			goto next;
+
 		dead_root = btrfs_read_fs_root_no_radix(root->fs_info, &key);
 		if (IS_ERR(dead_root)) {
 			ret = PTR_ERR(dead_root);
 			goto err;
 		}
-		ret = btrfs_add_dead_root(dead_root,
+
+		ret = btrfs_add_dead_root(dead_root, latest,
 					  &root->fs_info->dead_roots);
 		if (ret)
 			goto err;

fs/btrfs/transaction.c

@@ -239,7 +239,9 @@ struct dirty_root {
 	struct btrfs_root *latest_root;
 };
 
-int btrfs_add_dead_root(struct btrfs_root *root, struct list_head *dead_list)
+int btrfs_add_dead_root(struct btrfs_root *root,
+			struct btrfs_root *latest,
+			struct list_head *dead_list)
 {
 	struct dirty_root *dirty;
 
@@ -247,6 +249,7 @@ int btrfs_add_dead_root(struct btrfs_root *root, struct list_head *dead_list)
 	if (!dirty)
 		return -ENOMEM;
 	dirty->root = root;
+	dirty->latest_root = latest;
 	list_add(&dirty->list, dead_list);
 	return 0;
 }
@@ -412,7 +415,6 @@ static int drop_dirty_roots(struct btrfs_root *tree_root,
 
 		while(1) {
 			trans = btrfs_start_transaction(tree_root, 1);
-
 			ret = btrfs_drop_snapshot(trans, dirty->root);
 			if (ret != -EAGAIN) {
 				break;

fs/btrfs/transaction.h

@@ -78,7 +78,8 @@ void btrfs_transaction_flush_work(struct btrfs_root *root);
 void btrfs_transaction_queue_work(struct btrfs_root *root, int delay);
 void btrfs_init_transaction_sys(void);
 void btrfs_exit_transaction_sys(void);
-int btrfs_add_dead_root(struct btrfs_root *root, struct list_head *dead_list);
+int btrfs_add_dead_root(struct btrfs_root *root, struct btrfs_root *latest,
+			struct list_head *dead_list);
 int btrfs_defrag_dirty_roots(struct btrfs_fs_info *info);
 int btrfs_defrag_root(struct btrfs_root *root, int cacheonly);
 int btrfs_clean_old_snapshots(struct btrfs_root *root);