Walk list of processes attached to a shared page

Is it possible to walk the processes already attached to a shared page in an implementation of security_shm_shmat()? I have a function: static int my_shm_shmat(struct shmid_kernel *shp, char __user *shmaddr, int shmflag) and I would like to find the processes already attached. It seems like rmap_walk() would help, but I cannot figure out how to get an appropriate struct page out of a struct shmid_kernel or unmapped address (i.e., shmaddr above). -- Mike :wq