Btrfs: preftree: use rb_first_cached

rb_first_cached() trades an extra pointer "leftmost" for doing the same
job as rb_first() but in O(1).

While resolving indirect refs and missing refs, it always looks for the
first rb entry in a while loop, it's helpful to use rb_first_cached
instead.

For more details about the optimization see patch "Btrfs: delayed-refs:
use rb_first_cached for href_root".
Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Signed-off-by: Liu Bo <bo.liu@linux.alibaba.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/backref.c

@@ -112,11 +112,11 @@ static int find_extent_in_eb(const struct extent_buffer *eb,
 }
 
 struct preftree {
-	struct rb_root root;
+	struct rb_root_cached root;
 	unsigned int count;
 };
 
-#define PREFTREE_INIT	{ .root = RB_ROOT, .count = 0 }
+#define PREFTREE_INIT	{ .root = RB_ROOT_CACHED, .count = 0 }
 
 struct preftrees {
 	struct preftree direct;    /* BTRFS_SHARED_[DATA|BLOCK]_REF_KEY */
@@ -225,14 +225,15 @@ static void prelim_ref_insert(const struct btrfs_fs_info *fs_info,
 			      struct prelim_ref *newref,
 			      struct share_check *sc)
 {
-	struct rb_root *root;
+	struct rb_root_cached *root;
 	struct rb_node **p;
 	struct rb_node *parent = NULL;
 	struct prelim_ref *ref;
 	int result;
+	bool leftmost = true;
 
 	root = &preftree->root;
-	p = &root->rb_node;
+	p = &root->rb_root.rb_node;
 
 	while (*p) {
 		parent = *p;
@@ -242,6 +243,7 @@ static void prelim_ref_insert(const struct btrfs_fs_info *fs_info,
 			p = &(*p)->rb_left;
 		} else if (result > 0) {
 			p = &(*p)->rb_right;
+			leftmost = false;
 		} else {
 			/* Identical refs, merge them and free @newref */
 			struct extent_inode_elem *eie = ref->inode_list;
@@ -272,7 +274,7 @@ static void prelim_ref_insert(const struct btrfs_fs_info *fs_info,
 	preftree->count++;
 	trace_btrfs_prelim_ref_insert(fs_info, newref, NULL, preftree->count);
 	rb_link_node(&newref->rbnode, parent, p);
-	rb_insert_color(&newref->rbnode, root);
+	rb_insert_color_cached(&newref->rbnode, root, leftmost);
 }
 
 /*
@@ -283,11 +285,11 @@ static void prelim_release(struct preftree *preftree)
 {
 	struct prelim_ref *ref, *next_ref;
 
-	rbtree_postorder_for_each_entry_safe(ref, next_ref, &preftree->root,
-					     rbnode)
+	rbtree_postorder_for_each_entry_safe(ref, next_ref,
+					     &preftree->root.rb_root, rbnode)
 		free_pref(ref);
 
-	preftree->root = RB_ROOT;
+	preftree->root = RB_ROOT_CACHED;
 	preftree->count = 0;
 }
 
@@ -627,7 +629,7 @@ static int resolve_indirect_refs(struct btrfs_fs_info *fs_info,
 	 * freeing the entire indirect tree when we're done.  In some test
 	 * cases, the tree can grow quite large (~200k objects).
 	 */
-	while ((rnode = rb_first(&preftrees->indirect.root))) {
+	while ((rnode = rb_first_cached(&preftrees->indirect.root))) {
 		struct prelim_ref *ref;
 
 		ref = rb_entry(rnode, struct prelim_ref, rbnode);
@@ -637,7 +639,7 @@ static int resolve_indirect_refs(struct btrfs_fs_info *fs_info,
 			goto out;
 		}
 
-		rb_erase(&ref->rbnode, &preftrees->indirect.root);
+		rb_erase_cached(&ref->rbnode, &preftrees->indirect.root);
 		preftrees->indirect.count--;
 
 		if (ref->count == 0) {
@@ -717,9 +719,9 @@ static int add_missing_keys(struct btrfs_fs_info *fs_info,
 	struct preftree *tree = &preftrees->indirect_missing_keys;
 	struct rb_node *node;
 
-	while ((node = rb_first(&tree->root))) {
+	while ((node = rb_first_cached(&tree->root))) {
 		ref = rb_entry(node, struct prelim_ref, rbnode);
-		rb_erase(node, &tree->root);
+		rb_erase_cached(node, &tree->root);
 
 		BUG_ON(ref->parent);	/* should not be a direct ref */
 		BUG_ON(ref->key_for_search.type);
@@ -1229,14 +1231,14 @@ static int find_parent_nodes(struct btrfs_trans_handle *trans,
 	if (ret)
 		goto out;
 
-	WARN_ON(!RB_EMPTY_ROOT(&preftrees.indirect_missing_keys.root));
+	WARN_ON(!RB_EMPTY_ROOT(&preftrees.indirect_missing_keys.root.rb_root));
 
 	ret = resolve_indirect_refs(fs_info, path, time_seq, &preftrees,
 				    extent_item_pos, total_refs, sc, ignore_offset);
 	if (ret)
 		goto out;
 
-	WARN_ON(!RB_EMPTY_ROOT(&preftrees.indirect.root));
+	WARN_ON(!RB_EMPTY_ROOT(&preftrees.indirect.root.rb_root));
 
 	/*
 	 * This walks the tree of merged and resolved refs. Tree blocks are
@@ -1245,7 +1247,7 @@ static int find_parent_nodes(struct btrfs_trans_handle *trans,
 	 *
 	 * We release the entire tree in one go before returning.
 	 */
-	node = rb_first(&preftrees.direct.root);
+	node = rb_first_cached(&preftrees.direct.root);
 	while (node) {
 		ref = rb_entry(node, struct prelim_ref, rbnode);
 		node = rb_next(&ref->rbnode);