Tracing changes to mm->nr_ptes

Hi, As a learning exercise I'm using systemtap to trace all calls to pte_alloc() in the context of a particular process. pte_alloc() allocates a page table and increments mm->nr_ptes. However, the first time pte_alloc() is called (and the first time I can intercept it) mm->nr_ptes is already equal to 1. I cannot find the initial increment of mm->nr_ptes. Anyone know where I should look?! Thanks.