I want to add my variable in linux page cache

Question

I want to add my variable in linux page cache. Page cache is managed by radix tree.

struct radix_tree_node {
unsigned long X[RADIX_TREE_MAP_SIZE];
}

I want to add my variable 'X' in radix-tree-node. But, it doesn't work when fill 'X' of radix_tree_root.

There's an radix tree root named root. // struct radix_tree_root* root; root has rnode, which is a pointer of radix_tree_node.

root->rnode->X[offset] = number;

It fails, and I don't know why it doesn't work.