btrfs: backref, don't add refs from shared block when resolving normal backref

All references from the block of SHARED_DATA_REF belong to that shared
block backref.

For example:

  item 11 key (40831553536 EXTENT_ITEM 4194304) itemoff 15460 itemsize 95
      extent refs 24 gen 7302 flags DATA
      extent data backref root 257 objectid 260 offset 65536 count 5
      extent data backref root 258 objectid 265 offset 0 count 9
      shared data backref parent 394985472 count 10

Block 394985472 might be leaf from root 257, and the item obejctid and
(file_pos - file_extent_item::offset) in that leaf just happens to be
260 and 65536 which is equal to the first extent data backref entry.

Before this patch, when we resolve backref:

  root 257 objectid 260 offset 65536

we will add those refs in block 394985472 and wrongly treat those as the
refs we want.

Fix this by checking if the leaf we are processing is shared data
backref, if so, just skip this leaf.

Shared data refs added into preftrees.direct have all entry value = 0
(root_id = 0, key = NULL, level = 0) except parent entry.

Other refs from indirect tree will have key value and root id != 0, and
these values won't be changed when their parent is resolved and added to
preftrees.direct. Therefore, we could reuse the preftrees.direct and
search ref with all values = 0 except parent is set to avoid getting
those resolved refs block.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: ethanwu <ethanwu@synology.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/backref.c

@@ -386,8 +386,34 @@ static int add_indirect_ref(const struct btrfs_fs_info *fs_info,
 			      wanted_disk_byte, count, sc, gfp_mask);
 }
 
+static int is_shared_data_backref(struct preftrees *preftrees, u64 bytenr)
+{
+	struct rb_node **p = &preftrees->direct.root.rb_root.rb_node;
+	struct rb_node *parent = NULL;
+	struct prelim_ref *ref = NULL;
+	struct prelim_ref target = {0};
+	int result;
+
+	target.parent = bytenr;
+
+	while (*p) {
+		parent = *p;
+		ref = rb_entry(parent, struct prelim_ref, rbnode);
+		result = prelim_ref_compare(ref, &target);
+
+		if (result < 0)
+			p = &(*p)->rb_left;
+		else if (result > 0)
+			p = &(*p)->rb_right;
+		else
+			return 1;
+	}
+	return 0;
+}
+
 static int add_all_parents(struct btrfs_root *root, struct btrfs_path *path,
-			   struct ulist *parents, struct prelim_ref *ref,
+			   struct ulist *parents,
+			   struct preftrees *preftrees, struct prelim_ref *ref,
 			   int level, u64 time_seq, const u64 *extent_item_pos,
 			   u64 total_refs, bool ignore_offset)
 {
@@ -412,11 +438,16 @@ static int add_all_parents(struct btrfs_root *root, struct btrfs_path *path,
 	}
 
 	/*
-	 * We normally enter this function with the path already pointing to
+	 * 1. We normally enter this function with the path already pointing to
-	 * the first item to check. But sometimes, we may enter it with
+	 *    the first item to check. But sometimes, we may enter it with
-	 * slot==nritems. In that case, go to the next leaf before we continue.
+	 *    slot == nritems.
+	 * 2. We are searching for normal backref but bytenr of this leaf
+	 *    matches shared data backref
+	 * For these cases, go to the next leaf before we continue.
 	 */
-	if (path->slots[0] >= btrfs_header_nritems(path->nodes[0])) {
+	eb = path->nodes[0];
+	if (path->slots[0] >= btrfs_header_nritems(eb) ||
+	    is_shared_data_backref(preftrees, eb->start)) {
 		if (time_seq == SEQ_LAST)
 			ret = btrfs_next_leaf(root, path);
 		else
@@ -433,6 +464,17 @@ static int add_all_parents(struct btrfs_root *root, struct btrfs_path *path,
 		    key.type != BTRFS_EXTENT_DATA_KEY)
 			break;
 
+		/*
+		 * We are searching for normal backref but bytenr of this leaf
+		 * matches shared data backref.
+		 */
+		if (slot == 0 && is_shared_data_backref(preftrees, eb->start)) {
+			if (time_seq == SEQ_LAST)
+				ret = btrfs_next_leaf(root, path);
+			else
+				ret = btrfs_next_old_leaf(root, path, time_seq);
+			continue;
+		}
 		fi = btrfs_item_ptr(eb, slot, struct btrfs_file_extent_item);
 		disk_byte = btrfs_file_extent_disk_bytenr(eb, fi);
 		data_offset = btrfs_file_extent_offset(eb, fi);
@@ -484,6 +526,7 @@ next:
  */
 static int resolve_indirect_ref(struct btrfs_fs_info *fs_info,
 				struct btrfs_path *path, u64 time_seq,
+				struct preftrees *preftrees,
 				struct prelim_ref *ref, struct ulist *parents,
 				const u64 *extent_item_pos, u64 total_refs,
 				bool ignore_offset)
@@ -577,8 +620,8 @@ static int resolve_indirect_ref(struct btrfs_fs_info *fs_info,
 		eb = path->nodes[level];
 	}
 
-	ret = add_all_parents(root, path, parents, ref, level, time_seq,
+	ret = add_all_parents(root, path, parents, preftrees, ref, level,
-			      extent_item_pos, total_refs, ignore_offset);
+			      time_seq, extent_item_pos, total_refs, ignore_offset);
 out:
 	btrfs_put_root(root);
 out_free:
@@ -658,8 +701,8 @@ static int resolve_indirect_refs(struct btrfs_fs_info *fs_info,
 			ret = BACKREF_FOUND_SHARED;
 			goto out;
 		}
-		err = resolve_indirect_ref(fs_info, path, time_seq, ref,
+		err = resolve_indirect_ref(fs_info, path, time_seq, preftrees,
-					   parents, extent_item_pos,
+					   ref, parents, extent_item_pos,
 					   total_refs, ignore_offset);
 		/*
 		 * we can only tolerate ENOENT,otherwise,we should catch error