btrfs: reloc: use btrfs_backref_iter infrastructure

In the core function of relocation, build_backref_tree, it needs to
iterate all backref items of one tree block.

Use btrfs_backref_iter infrastructure to do the loop and make the code
more readable.

The backref items look would be much more easier to read:

	ret = btrfs_backref_iter_start(iter, cur->bytenr);
	for (; ret == 0; ret = btrfs_backref_iter_next(iter)) {
		/* The really important work */
	}
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/relocation.c

@@ -654,48 +654,6 @@ static struct btrfs_root *read_fs_root(struct btrfs_fs_info *fs_info,
 	return btrfs_get_fs_root(fs_info, &key, false);
 }
 
-static noinline_for_stack
-int find_inline_backref(struct extent_buffer *leaf, int slot,
-			unsigned long *ptr, unsigned long *end)
-{
-	struct btrfs_key key;
-	struct btrfs_extent_item *ei;
-	struct btrfs_tree_block_info *bi;
-	u32 item_size;
-
-	btrfs_item_key_to_cpu(leaf, &key, slot);
-
-	item_size = btrfs_item_size_nr(leaf, slot);
-	if (item_size < sizeof(*ei)) {
-		btrfs_print_v0_err(leaf->fs_info);
-		btrfs_handle_fs_error(leaf->fs_info, -EINVAL, NULL);
-		return 1;
-	}
-	ei = btrfs_item_ptr(leaf, slot, struct btrfs_extent_item);
-	WARN_ON(!(btrfs_extent_flags(leaf, ei) &
-		  BTRFS_EXTENT_FLAG_TREE_BLOCK));
-
-	if (key.type == BTRFS_EXTENT_ITEM_KEY &&
-	    item_size <= sizeof(*ei) + sizeof(*bi)) {
-		WARN_ON(item_size < sizeof(*ei) + sizeof(*bi));
-		return 1;
-	}
-	if (key.type == BTRFS_METADATA_ITEM_KEY &&
-	    item_size <= sizeof(*ei)) {
-		WARN_ON(item_size < sizeof(*ei));
-		return 1;
-	}
-
-	if (key.type == BTRFS_EXTENT_ITEM_KEY) {
-		bi = (struct btrfs_tree_block_info *)(ei + 1);
-		*ptr = (unsigned long)(bi + 1);
-	} else {
-		*ptr = (unsigned long)(ei + 1);
-	}
-	*end = (unsigned long)ei + item_size;
-	return 0;
-}
-
 /*
  * build backref tree for a given tree block. root of the backref tree
  * corresponds the tree block, leaves of the backref tree correspond
@@ -715,10 +673,10 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 					struct btrfs_key *node_key,
 					int level, u64 bytenr)
 {
+	struct btrfs_backref_iter *iter;
 	struct backref_cache *cache = &rc->backref_cache;
-	struct btrfs_path *path1; /* For searching extent root */
-	struct btrfs_path *path2; /* For searching parent of TREE_BLOCK_REF */
-	struct extent_buffer *eb;
+	/* For searching parent of TREE_BLOCK_REF */
+	struct btrfs_path *path;
 	struct btrfs_root *root;
 	struct backref_node *cur;
 	struct backref_node *upper;
@@ -727,9 +685,6 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 	struct backref_node *exist = NULL;
 	struct backref_edge *edge;
 	struct rb_node *rb_node;
-	struct btrfs_key key;
-	unsigned long end;
-	unsigned long ptr;
 	LIST_HEAD(list); /* Pending edge list, upper node needs to be checked */
 	LIST_HEAD(useless);
 	int cowonly;
@@ -737,9 +692,11 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 	int err = 0;
 	bool need_check = true;
 
-	path1 = btrfs_alloc_path();
-	path2 = btrfs_alloc_path();
-	if (!path1 || !path2) {
+	iter = btrfs_backref_iter_alloc(rc->extent_root->fs_info, GFP_NOFS);
+	if (!iter)
+		return ERR_PTR(-ENOMEM);
+	path = btrfs_alloc_path();
+	if (!path) {
 		err = -ENOMEM;
 		goto out;
 	}
@@ -755,25 +712,28 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 	node->lowest = 1;
 	cur = node;
 again:
-	end = 0;
-	ptr = 0;
-	key.objectid = cur->bytenr;
-	key.type = BTRFS_METADATA_ITEM_KEY;
-	key.offset = (u64)-1;
-
-	path1->search_commit_root = 1;
-	path1->skip_locking = 1;
-	ret = btrfs_search_slot(NULL, rc->extent_root, &key, path1,
-				0, 0);
+	ret = btrfs_backref_iter_start(iter, cur->bytenr);
 	if (ret < 0) {
 		err = ret;
 		goto out;
 	}
-	ASSERT(ret);
-	ASSERT(path1->slots[0]);
-
-	path1->slots[0]--;
 
+	/*
+	 * We skip the first btrfs_tree_block_info, as we don't use the key
+	 * stored in it, but fetch it from the tree block
+	 */
+	if (btrfs_backref_has_tree_block_info(iter)) {
+		ret = btrfs_backref_iter_next(iter);
+		if (ret < 0) {
+			err = ret;
+			goto out;
+		}
+		/* No extra backref? This means the tree block is corrupted */
+		if (ret > 0) {
+			err = -EUCLEAN;
+			goto out;
+		}
+	}
 	WARN_ON(cur->checked);
 	if (!list_empty(&cur->upper)) {
 		/*
@@ -795,42 +755,21 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 		exist = NULL;
 	}
 
-	while (1) {
-		cond_resched();
-		eb = path1->nodes[0];
-
-		if (ptr >= end) {
-			if (path1->slots[0] >= btrfs_header_nritems(eb)) {
-				ret = btrfs_next_leaf(rc->extent_root, path1);
-				if (ret < 0) {
-					err = ret;
-					goto out;
-				}
-				if (ret > 0)
-					break;
-				eb = path1->nodes[0];
-			}
+	for (; ret == 0; ret = btrfs_backref_iter_next(iter)) {
+		struct extent_buffer *eb;
+		struct btrfs_key key;
+		int type;
 
-			btrfs_item_key_to_cpu(eb, &key, path1->slots[0]);
-			if (key.objectid != cur->bytenr) {
-				WARN_ON(exist);
-				break;
-			}
+		cond_resched();
+		eb = btrfs_backref_get_eb(iter);
 
-			if (key.type == BTRFS_EXTENT_ITEM_KEY ||
-			    key.type == BTRFS_METADATA_ITEM_KEY) {
-				ret = find_inline_backref(eb, path1->slots[0],
-							  &ptr, &end);
-				if (ret)
-					goto next;
-			}
-		}
+		key.objectid = iter->bytenr;
+		if (btrfs_backref_iter_is_inline_ref(iter)) {
+			struct btrfs_extent_inline_ref *iref;
 
-		if (ptr < end) {
 			/* update key for inline back ref */
-			struct btrfs_extent_inline_ref *iref;
-			int type;
-			iref = (struct btrfs_extent_inline_ref *)ptr;
+			iref = (struct btrfs_extent_inline_ref *)
+				((unsigned long)iter->cur_ptr);
 			type = btrfs_get_extent_inline_ref_type(eb, iref,
 							BTRFS_REF_TYPE_BLOCK);
 			if (type == BTRFS_REF_TYPE_INVALID) {
@@ -839,9 +778,9 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 			}
 			key.type = type;
 			key.offset = btrfs_extent_inline_ref_offset(eb, iref);
-
-			WARN_ON(key.type != BTRFS_TREE_BLOCK_REF_KEY &&
-				key.type != BTRFS_SHARED_BLOCK_REF_KEY);
+		} else {
+			key.type = iter->cur_key.type;
+			key.offset = iter->cur_key.offset;
 		}
 
 		/*
@@ -854,7 +793,7 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 		     (key.type == BTRFS_SHARED_BLOCK_REF_KEY &&
 		      exist->bytenr == key.offset))) {
 			exist = NULL;
-			goto next;
+			continue;
 		}
 
 		/* SHARED_BLOCK_REF means key.offset is the parent bytenr */
@@ -900,7 +839,7 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 			edge->node[LOWER] = cur;
 			edge->node[UPPER] = upper;
 
-			goto next;
+			continue;
 		} else if (unlikely(key.type == BTRFS_EXTENT_REF_V0_KEY)) {
 			err = -EINVAL;
 			btrfs_print_v0_err(rc->extent_root->fs_info);
@@ -908,7 +847,7 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 					      NULL);
 			goto out;
 		} else if (key.type != BTRFS_TREE_BLOCK_REF_KEY) {
-			goto next;
+			continue;
 		}
 
 		/*
@@ -941,21 +880,21 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 		level = cur->level + 1;
 
 		/* Search the tree to find parent blocks referring the block. */
-		path2->search_commit_root = 1;
-		path2->skip_locking = 1;
-		path2->lowest_level = level;
-		ret = btrfs_search_slot(NULL, root, node_key, path2, 0, 0);
-		path2->lowest_level = 0;
+		path->search_commit_root = 1;
+		path->skip_locking = 1;
+		path->lowest_level = level;
+		ret = btrfs_search_slot(NULL, root, node_key, path, 0, 0);
+		path->lowest_level = 0;
 		if (ret < 0) {
 			btrfs_put_root(root);
 			err = ret;
 			goto out;
 		}
-		if (ret > 0 && path2->slots[level] > 0)
-			path2->slots[level]--;
+		if (ret > 0 && path->slots[level] > 0)
+			path->slots[level]--;
 
-		eb = path2->nodes[level];
-		if (btrfs_node_blockptr(eb, path2->slots[level]) !=
+		eb = path->nodes[level];
+		if (btrfs_node_blockptr(eb, path->slots[level]) !=
 		    cur->bytenr) {
 			btrfs_err(root->fs_info,
 	"couldn't find block (%llu) (level %d) in tree (%llu) with key (%llu %u %llu)",
@@ -972,7 +911,7 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 
 		/* Add all nodes and edges in the path */
 		for (; level < BTRFS_MAX_LEVEL; level++) {
-			if (!path2->nodes[level]) {
+			if (!path->nodes[level]) {
 				ASSERT(btrfs_root_bytenr(&root->root_item) ==
 				       lower->bytenr);
 				if (should_ignore_root(root)) {
@@ -991,7 +930,7 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 				goto out;
 			}
 
-			eb = path2->nodes[level];
+			eb = path->nodes[level];
 			rb_node = tree_search(&cache->rb_root, eb->start);
 			if (!rb_node) {
 				upper = alloc_backref_node(cache);
@@ -1051,20 +990,14 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 			lower = upper;
 			upper = NULL;
 		}
-		btrfs_release_path(path2);
-next:
-		if (ptr < end) {
-			ptr += btrfs_extent_inline_ref_size(key.type);
-			if (ptr >= end) {
-				WARN_ON(ptr > end);
-				ptr = 0;
-				end = 0;
-			}
-		}
-		if (ptr >= end)
-			path1->slots[0]++;
+		btrfs_release_path(path);
+	}
+	if (ret < 0) {
+		err = ret;
+		goto out;
 	}
-	btrfs_release_path(path1);
+	ret = 0;
+	btrfs_backref_iter_release(iter);
 
 	cur->checked = 1;
 	WARN_ON(exist);
@@ -1182,8 +1115,8 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 		}
 	}
 out:
-	btrfs_free_path(path1);
-	btrfs_free_path(path2);
+	btrfs_backref_iter_free(iter);
+	btrfs_free_path(path);
 	if (err) {
 		while (!list_empty(&useless)) {
 			lower = list_entry(useless.next,