+/*
+ * check if the tree block can be shared by multiple trees
+ */
+static int btrfs_block_can_be_shared(struct btrfs_root *root,
+ struct extent_buffer *buf)
+{
+ /*
+ * Tree blocks not in reference counted trees and tree roots
+ * are never shared. If a block was allocated after the last
+ * snapshot and the block was not allocated by tree relocation,
+ * we know the block is not shared.
+ */
+ if (root->ref_cows &&
+ buf != root->node && buf != root->commit_root &&
+ (btrfs_header_generation(buf) <=
+ btrfs_root_last_snapshot(&root->root_item) ||
+ btrfs_header_flag(buf, BTRFS_HEADER_FLAG_RELOC)))
+ return 1;
+#ifdef BTRFS_COMPAT_EXTENT_TREE_V0
+ if (root->ref_cows &&
+ btrfs_header_backref_rev(buf) < BTRFS_MIXED_BACKREF_REV)
+ return 1;
+#endif
+ return 0;
+}
+
+static noinline int update_ref_for_cow(struct btrfs_trans_handle *trans,
+ struct btrfs_root *root,
+ struct extent_buffer *buf,
+ struct extent_buffer *cow)
+{
+ u64 refs;
+ u64 owner;
+ u64 flags;
+ u64 new_flags = 0;
+ int ret;
+
+ /*
+ * Backrefs update rules:
+ *
+ * Always use full backrefs for extent pointers in tree block
+ * allocated by tree relocation.
+ *
+ * If a shared tree block is no longer referenced by its owner
+ * tree (btrfs_header_owner(buf) == root->root_key.objectid),
+ * use full backrefs for extent pointers in tree block.
+ *
+ * If a tree block is been relocating
+ * (root->root_key.objectid == BTRFS_TREE_RELOC_OBJECTID),
+ * use full backrefs for extent pointers in tree block.
+ * The reason for this is some operations (such as drop tree)
+ * are only allowed for blocks use full backrefs.
+ */
+
+ if (btrfs_block_can_be_shared(root, buf)) {
+ ret = btrfs_lookup_extent_info(trans, root, buf->start,
+ btrfs_header_level(buf), 1,
+ &refs, &flags);
+ BUG_ON(ret);
+ BUG_ON(refs == 0);
+ } else {
+ refs = 1;
+ if (root->root_key.objectid == BTRFS_TREE_RELOC_OBJECTID ||
+ btrfs_header_backref_rev(buf) < BTRFS_MIXED_BACKREF_REV)
+ flags = BTRFS_BLOCK_FLAG_FULL_BACKREF;
+ else
+ flags = 0;
+ }
+
+ owner = btrfs_header_owner(buf);
+ BUG_ON(!(flags & BTRFS_BLOCK_FLAG_FULL_BACKREF) &&
+ owner == BTRFS_TREE_RELOC_OBJECTID);
+
+ if (refs > 1) {
+ if ((owner == root->root_key.objectid ||
+ root->root_key.objectid == BTRFS_TREE_RELOC_OBJECTID) &&
+ !(flags & BTRFS_BLOCK_FLAG_FULL_BACKREF)) {
+ ret = btrfs_inc_ref(trans, root, buf, 1);
+ BUG_ON(ret);
+
+ if (root->root_key.objectid ==
+ BTRFS_TREE_RELOC_OBJECTID) {
+ ret = btrfs_dec_ref(trans, root, buf, 0);
+ BUG_ON(ret);
+ ret = btrfs_inc_ref(trans, root, cow, 1);
+ BUG_ON(ret);
+ }
+ new_flags |= BTRFS_BLOCK_FLAG_FULL_BACKREF;
+ } else {
+
+ if (root->root_key.objectid ==
+ BTRFS_TREE_RELOC_OBJECTID)
+ ret = btrfs_inc_ref(trans, root, cow, 1);
+ else
+ ret = btrfs_inc_ref(trans, root, cow, 0);
+ BUG_ON(ret);
+ }
+ if (new_flags != 0) {
+ ret = btrfs_set_block_flags(trans, root, buf->start,
+ btrfs_header_level(buf),
+ new_flags);
+ BUG_ON(ret);
+ }
+ } else {
+ if (flags & BTRFS_BLOCK_FLAG_FULL_BACKREF) {
+ if (root->root_key.objectid ==
+ BTRFS_TREE_RELOC_OBJECTID)
+ ret = btrfs_inc_ref(trans, root, cow, 1);
+ else
+ ret = btrfs_inc_ref(trans, root, cow, 0);
+ BUG_ON(ret);
+ ret = btrfs_dec_ref(trans, root, buf, 1);
+ BUG_ON(ret);
+ }
+ clean_tree_block(trans, root, buf);
+ }
+ return 0;
+}
+