btrfs: don't build backref tree for COW-only blocks

We already determine the owner for any blocks we find when we're
relocating, and for COW-only blocks (and the data reloc tree) we COW
down to the block and call it good enough.  However we still build a
whole backref tree for them, even though we're not going to use it, and
then just don't put these blocks in the cache.

Rework the code to check if the block belongs to a COW-only root or the
data reloc root, and then just cow down to the block, skipping the
backref cache generation.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 73 insertions, 19 deletions

diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index cb98ddd825d4..c8f35d456a61 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -2133,17 +2133,11 @@ static noinline_for_stack u64 calcu_metadata_size(struct reloc_control *rc,
 	return num_bytes;
 }
 
-static int reserve_metadata_space(struct btrfs_trans_handle *trans,
-				  struct reloc_control *rc,
-				  struct btrfs_backref_node *node)
+static int refill_metadata_space(struct btrfs_trans_handle *trans,
+				 struct reloc_control *rc, u64 num_bytes)
 {
-	struct btrfs_root *root = rc->extent_root;
-	struct btrfs_fs_info *fs_info = root->fs_info;
-	u64 num_bytes;
+	struct btrfs_fs_info *fs_info = trans->fs_info;
 	int ret;
-	u64 tmp;
-
-	num_bytes = calcu_metadata_size(rc, node) * 2;
 
 	trans->block_rsv = rc->block_rsv;
 	rc->reserved_bytes += num_bytes;
@@ -2156,7 +2150,8 @@ static int reserve_metadata_space(struct btrfs_trans_handle *trans,
 	ret = btrfs_block_rsv_refill(fs_info, rc->block_rsv, num_bytes,
 				     BTRFS_RESERVE_FLUSH_LIMIT);
 	if (ret) {
-		tmp = fs_info->nodesize * RELOCATION_RESERVED_NODES;
+		u64 tmp = fs_info->nodesize * RELOCATION_RESERVED_NODES;
+
 		while (tmp <= rc->reserved_bytes)
 			tmp <<= 1;
 		/*
@@ -2174,6 +2169,16 @@ static int reserve_metadata_space(struct btrfs_trans_handle *trans,
 	return 0;
 }
 
+static int reserve_metadata_space(struct btrfs_trans_handle *trans,
+				  struct reloc_control *rc,
+				  struct btrfs_backref_node *node)
+{
+	u64 num_bytes;
+
+	num_bytes = calcu_metadata_size(rc, node) * 2;
+	return refill_metadata_space(trans, rc, num_bytes);
+}
+
 /*
  * relocate a block tree, and then update pointers in upper level
  * blocks that reference the block to point to the new location.
@@ -2525,15 +2530,11 @@ static int relocate_tree_block(struct btrfs_trans_handle *trans,
 			node->root = btrfs_grab_root(root);
 			ASSERT(node->root);
 		} else {
-			path->lowest_level = node->level;
-			if (root == root->fs_info->chunk_root)
-				btrfs_reserve_chunk_metadata(trans, false);
-			ret = btrfs_search_slot(trans, root, key, path, 0, 1);
-			btrfs_release_path(path);
-			if (root == root->fs_info->chunk_root)
-				btrfs_trans_release_chunk_metadata(trans);
-			if (ret > 0)
-				ret = 0;
+			btrfs_err(root->fs_info,
+				  "bytenr %llu resolved to a non-shareable root",
+				  node->bytenr);
+			ret = -EUCLEAN;
+			goto out;
 		}
 		if (!ret)
 			update_processed_blocks(rc, node);
@@ -2546,6 +2547,45 @@ out:
 	return ret;
 }
 
+static int relocate_cowonly_block(struct btrfs_trans_handle *trans,
+				  struct reloc_control *rc, struct tree_block *block,
+				  struct btrfs_path *path)
+{
+	struct btrfs_fs_info *fs_info = trans->fs_info;
+	struct btrfs_root *root;
+	u64 num_bytes;
+	int nr_levels;
+	int ret;
+
+	root = btrfs_get_fs_root(fs_info, block->owner, true);
+	if (IS_ERR(root))
+		return PTR_ERR(root);
+
+	nr_levels = max(btrfs_header_level(root->node) - block->level, 0) + 1;
+
+	num_bytes = fs_info->nodesize * nr_levels;
+	ret = refill_metadata_space(trans, rc, num_bytes);
+	if (ret) {
+		btrfs_put_root(root);
+		return ret;
+	}
+	path->lowest_level = block->level;
+	if (root == root->fs_info->chunk_root)
+		btrfs_reserve_chunk_metadata(trans, false);
+
+	ret = btrfs_search_slot(trans, root, &block->key, path, 0, 1);
+	path->lowest_level = 0;
+	btrfs_release_path(path);
+
+	if (root == root->fs_info->chunk_root)
+		btrfs_trans_release_chunk_metadata(trans);
+	if (ret > 0)
+		ret = 0;
+	btrfs_put_root(root);
+
+	return ret;
+}
+
 /*
  * relocate a list of blocks
  */
@@ -2585,6 +2625,20 @@ int relocate_tree_blocks(struct btrfs_trans_handle *trans,
 
 	/* Do tree relocation */
 	rbtree_postorder_for_each_entry_safe(block, next, blocks, rb_node) {
+		/*
+		 * For COWonly blocks, or the data reloc tree, we only need to
+		 * COW down to the block, there's no need to generate a backref
+		 * tree.
+		 */
+		if (block->owner &&
+		    (!is_fstree(block->owner) ||
+		     block->owner == BTRFS_DATA_RELOC_TREE_OBJECTID)) {
+			ret = relocate_cowonly_block(trans, rc, block, path);
+			if (ret)
+				break;
+			continue;
+		}
+
 		node = build_backref_tree(trans, rc, &block->key,
 					  block->level, block->bytenr);
 		if (IS_ERR(node)) {