some questions about container_of and user_struct

[miloody@gmail.com (loody)]

hi all:
I add below function in free_uid to get which task try to free user_struct

struct task_struct *p;
struct cred *c;
c = container_of(up,struct cred, user);
p = container_of(c,struct task_struct,real_cred);
printk("%s user %p, cred->user = %p p->pid = %d\n",__func__,up, c->user,p->pid);

but p->pid is always 0.
I think it is wrong for me to get cred and task by using container_of.

1. from kernel definition below, is it ok that member is pointer?
#define container_of(ptr, type, member)

2. is there already exist macros or functions I can use for #1 above?

3. what is user_struct used for? When and under what circumstances
kernel will try to release it.

4. since user_struct is allocated by kmem_cache_zalloc, is there api
or tool I can monitor it?

Thanks for your help,