Btrfs: speed up snapshot dropping

This patch contains two changes to avoid unnecessary tree block reads during
snapshot dropping.

First, check tree block's reference count and flags before reading the tree
block. if reference count > 1 and there is no need to update backrefs, we can
avoid reading the tree block.

Second, save when snapshot was created in root_key.offset. we can compare block
pointer's generation with snapshot's creation generation during updating
backrefs. If a given block was created before snapshot was created, the
snapshot can't be the tree block's owner. So we can avoid reading the block.
Signed-off-by: Yan Zheng <zheng.yan@oracle.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/extent-tree.c

@@ -4886,19 +4886,90 @@ struct walk_control {
 	int shared_level;
 	int update_ref;
 	int keep_locks;
+	int reada_slot;
+	int reada_count;
 };
 
 #define DROP_REFERENCE	1
 #define UPDATE_BACKREF	2
 
+static noinline void reada_walk_down(struct btrfs_trans_handle *trans,
+				     struct btrfs_root *root,
+				     struct walk_control *wc,
+				     struct btrfs_path *path)
+{
+	u64 bytenr;
+	u64 generation;
+	u64 refs;
+	u64 last = 0;
+	u32 nritems;
+	u32 blocksize;
+	struct btrfs_key key;
+	struct extent_buffer *eb;
+	int ret;
+	int slot;
+	int nread = 0;
+
+	if (path->slots[wc->level] < wc->reada_slot) {
+		wc->reada_count = wc->reada_count * 2 / 3;
+		wc->reada_count = max(wc->reada_count, 2);
+	} else {
+		wc->reada_count = wc->reada_count * 3 / 2;
+		wc->reada_count = min_t(int, wc->reada_count,
+					BTRFS_NODEPTRS_PER_BLOCK(root));
+	}
+
+	eb = path->nodes[wc->level];
+	nritems = btrfs_header_nritems(eb);
+	blocksize = btrfs_level_size(root, wc->level - 1);
+
+	for (slot = path->slots[wc->level]; slot < nritems; slot++) {
+		if (nread >= wc->reada_count)
+			break;
+
+		cond_resched();
+		bytenr = btrfs_node_blockptr(eb, slot);
+		generation = btrfs_node_ptr_generation(eb, slot);
+
+		if (slot == path->slots[wc->level])
+			goto reada;
+
+		if (wc->stage == UPDATE_BACKREF &&
+		    generation <= root->root_key.offset)
+			continue;
+
+		if (wc->stage == DROP_REFERENCE) {
+			ret = btrfs_lookup_extent_info(trans, root,
+						bytenr, blocksize,
+						&refs, NULL);
+			BUG_ON(ret);
+			BUG_ON(refs == 0);
+			if (refs == 1)
+				goto reada;
+
+			if (!wc->update_ref ||
+			    generation <= root->root_key.offset)
+				continue;
+			btrfs_node_key_to_cpu(eb, &key, slot);
+			ret = btrfs_comp_cpu_keys(&key,
+						  &wc->update_progress);
+			if (ret < 0)
+				continue;
+		}
+reada:
+		ret = readahead_tree_block(root, bytenr, blocksize,
+					   generation);
+		if (ret)
+			break;
+		last = bytenr + blocksize;
+		nread++;
+	}
+	wc->reada_slot = slot;
+}
+
 /*
  * hepler to process tree block while walking down the tree.
  *
- * when wc->stage == DROP_REFERENCE, this function checks
- * reference count of the block. if the block is shared and
- * we need update back refs for the subtree rooted at the
- * block, this function changes wc->stage to UPDATE_BACKREF
- *
  * when wc->stage == UPDATE_BACKREF, this function updates
  * back refs for pointers in the block.
  *
@@ -4911,7 +4982,6 @@ static noinline int walk_down_proc(struct btrfs_trans_handle *trans,
 {
 	int level = wc->level;
 	struct extent_buffer *eb = path->nodes[level];
-	struct btrfs_key key;
 	u64 flag = BTRFS_BLOCK_FLAG_FULL_BACKREF;
 	int ret;
 
@@ -4934,21 +5004,6 @@ static noinline int walk_down_proc(struct btrfs_trans_handle *trans,
 		BUG_ON(wc->refs[level] == 0);
 	}
 
-	if (wc->stage == DROP_REFERENCE &&
-	    wc->update_ref && wc->refs[level] > 1) {
-		BUG_ON(eb == root->node);
-		BUG_ON(path->slots[level] > 0);
-		if (level == 0)
-			btrfs_item_key_to_cpu(eb, &key, path->slots[level]);
-		else
-			btrfs_node_key_to_cpu(eb, &key, path->slots[level]);
-		if (btrfs_header_owner(eb) == root->root_key.objectid &&
-		    btrfs_comp_cpu_keys(&key, &wc->update_progress) >= 0) {
-			wc->stage = UPDATE_BACKREF;
-			wc->shared_level = level;
-		}
-	}
-
 	if (wc->stage == DROP_REFERENCE) {
 		if (wc->refs[level] > 1)
 			return 1;
@@ -4984,6 +5039,123 @@ static noinline int walk_down_proc(struct btrfs_trans_handle *trans,
 	return 0;
 }
 
+/*
+ * hepler to process tree block pointer.
+ *
+ * when wc->stage == DROP_REFERENCE, this function checks
+ * reference count of the block pointed to. if the block
+ * is shared and we need update back refs for the subtree
+ * rooted at the block, this function changes wc->stage to
+ * UPDATE_BACKREF. if the block is shared and there is no
+ * need to update back, this function drops the reference
+ * to the block.
+ *
+ * NOTE: return value 1 means we should stop walking down.
+ */
+static noinline int do_walk_down(struct btrfs_trans_handle *trans,
+				 struct btrfs_root *root,
+				 struct btrfs_path *path,
+				 struct walk_control *wc)
+{
+	u64 bytenr;
+	u64 generation;
+	u64 parent;
+	u32 blocksize;
+	struct btrfs_key key;
+	struct extent_buffer *next;
+	int level = wc->level;
+	int reada = 0;
+	int ret = 0;
+
+	generation = btrfs_node_ptr_generation(path->nodes[level],
+					       path->slots[level]);
+	/*
+	 * if the lower level block was created before the snapshot
+	 * was created, we know there is no need to update back refs
+	 * for the subtree
+	 */
+	if (wc->stage == UPDATE_BACKREF &&
+	    generation <= root->root_key.offset)
+		return 1;
+
+	bytenr = btrfs_node_blockptr(path->nodes[level], path->slots[level]);
+	blocksize = btrfs_level_size(root, level - 1);
+
+	next = btrfs_find_tree_block(root, bytenr, blocksize);
+	if (!next) {
+		next = btrfs_find_create_tree_block(root, bytenr, blocksize);
+		reada = 1;
+	}
+	btrfs_tree_lock(next);
+	btrfs_set_lock_blocking(next);
+
+	if (wc->stage == DROP_REFERENCE) {
+		ret = btrfs_lookup_extent_info(trans, root, bytenr, blocksize,
+					       &wc->refs[level - 1],
+					       &wc->flags[level - 1]);
+		BUG_ON(ret);
+		BUG_ON(wc->refs[level - 1] == 0);
+
+		if (wc->refs[level - 1] > 1) {
+			if (!wc->update_ref ||
+			    generation <= root->root_key.offset)
+				goto skip;
+
+			btrfs_node_key_to_cpu(path->nodes[level], &key,
+					      path->slots[level]);
+			ret = btrfs_comp_cpu_keys(&key, &wc->update_progress);
+			if (ret < 0)
+				goto skip;
+
+			wc->stage = UPDATE_BACKREF;
+			wc->shared_level = level - 1;
+		}
+	}
+
+	if (!btrfs_buffer_uptodate(next, generation)) {
+		btrfs_tree_unlock(next);
+		free_extent_buffer(next);
+		next = NULL;
+	}
+
+	if (!next) {
+		if (reada && level == 1)
+			reada_walk_down(trans, root, wc, path);
+		next = read_tree_block(root, bytenr, blocksize, generation);
+		btrfs_tree_lock(next);
+		btrfs_set_lock_blocking(next);
+	}
+
+	level--;
+	BUG_ON(level != btrfs_header_level(next));
+	path->nodes[level] = next;
+	path->slots[level] = 0;
+	path->locks[level] = 1;
+	wc->level = level;
+	if (wc->level == 1)
+		wc->reada_slot = 0;
+	return 0;
+skip:
+	wc->refs[level - 1] = 0;
+	wc->flags[level - 1] = 0;
+
+	if (wc->flags[level] & BTRFS_BLOCK_FLAG_FULL_BACKREF) {
+		parent = path->nodes[level]->start;
+	} else {
+		BUG_ON(root->root_key.objectid !=
+		       btrfs_header_owner(path->nodes[level]));
+		parent = 0;
+	}
+
+	ret = btrfs_free_extent(trans, root, bytenr, blocksize, parent,
+				root->root_key.objectid, level - 1, 0);
+	BUG_ON(ret);
+
+	btrfs_tree_unlock(next);
+	free_extent_buffer(next);
+	return 1;
+}
+
 /*
  * hepler to process tree block while walking up the tree.
  *
@@ -5011,7 +5183,6 @@ static noinline int walk_up_proc(struct btrfs_trans_handle *trans,
 		if (level < wc->shared_level)
 			goto out;
 
-		BUG_ON(wc->refs[level] <= 1);
 		ret = find_next_key(path, level + 1, &wc->update_progress);
 		if (ret > 0)
 			wc->update_ref = 0;
@@ -5042,8 +5213,6 @@ static noinline int walk_up_proc(struct btrfs_trans_handle *trans,
 				path->locks[level] = 0;
 				return 1;
 			}
-		} else {
-			BUG_ON(level != 0);
 		}
 	}
 
@@ -5096,17 +5265,13 @@ static noinline int walk_down_tree(struct btrfs_trans_handle *trans,
 				   struct btrfs_path *path,
 				   struct walk_control *wc)
 {
-	struct extent_buffer *next;
-	struct extent_buffer *cur;
-	u64 bytenr;
-	u64 ptr_gen;
-	u32 blocksize;
 	int level = wc->level;
 	int ret;
 
 	while (level >= 0) {
-		cur = path->nodes[level];
-		BUG_ON(path->slots[level] >= btrfs_header_nritems(cur));
+		if (path->slots[level] >=
+		    btrfs_header_nritems(path->nodes[level]))
+			break;
 
 		ret = walk_down_proc(trans, root, path, wc);
 		if (ret > 0)
@@ -5115,20 +5280,12 @@ static noinline int walk_down_tree(struct btrfs_trans_handle *trans,
 		if (level == 0)
 			break;
 
-		bytenr = btrfs_node_blockptr(cur, path->slots[level]);
-		blocksize = btrfs_level_size(root, level - 1);
-		ptr_gen = btrfs_node_ptr_generation(cur, path->slots[level]);
-
-		next = read_tree_block(root, bytenr, blocksize, ptr_gen);
-		btrfs_tree_lock(next);
-		btrfs_set_lock_blocking(next);
-
-		level--;
-		BUG_ON(level != btrfs_header_level(next));
-		path->nodes[level] = next;
-		path->slots[level] = 0;
-		path->locks[level] = 1;
-		wc->level = level;
+		ret = do_walk_down(trans, root, path, wc);
+		if (ret > 0) {
+			path->slots[level]++;
+			continue;
+		}
+		level = wc->level;
 	}
 	return 0;
 }
@@ -5218,9 +5375,7 @@ int btrfs_drop_snapshot(struct btrfs_root *root, int update_ref)
 			err = ret;
 			goto out;
 		}
-		btrfs_node_key_to_cpu(path->nodes[level], &key,
-				      path->slots[level]);
-		WARN_ON(memcmp(&key, &wc->update_progress, sizeof(key)));
+		WARN_ON(ret > 0);
 
 		/*
 		 * unlock our path, this is safe because only this
@@ -5255,6 +5410,7 @@ int btrfs_drop_snapshot(struct btrfs_root *root, int update_ref)
 	wc->stage = DROP_REFERENCE;
 	wc->update_ref = update_ref;
 	wc->keep_locks = 0;
+	wc->reada_count = BTRFS_NODEPTRS_PER_BLOCK(root);
 
 	while (1) {
 		ret = walk_down_tree(trans, root, path, wc);
@@ -5361,6 +5517,7 @@ int btrfs_drop_subtree(struct btrfs_trans_handle *trans,
 	wc->stage = DROP_REFERENCE;
 	wc->update_ref = 0;
 	wc->keep_locks = 1;
+	wc->reada_count = BTRFS_NODEPTRS_PER_BLOCK(root);
 
 	while (1) {
 		wret = walk_down_tree(trans, root, path, wc);

fs/btrfs/transaction.c

@@ -720,7 +720,8 @@ static noinline int create_pending_snapshot(struct btrfs_trans_handle *trans,
 	memcpy(new_root_item, &root->root_item, sizeof(*new_root_item));
 
 	key.objectid = objectid;
-	key.offset = 0;
+	/* record when the snapshot was created in key.offset */
+	key.offset = trans->transid;
 	btrfs_set_key_type(&key, BTRFS_ROOT_ITEM_KEY);
 
 	old = btrfs_lock_root_node(root);