copy_user_page
extern void copy_user_page(void *vto, void *vfrom, unsigned long vaddr,
extern void copy_user_page(void *to, void *from, unsigned long vaddr,
void copy_user_page(void *to, void *from, unsigned long vaddr, struct page *topage);
copy_user_page(vto, vfrom, vaddr, to);
copy_user_page(vto, kaddr, vmf->address, vmf->cow_page);
copy_user_page(vto, vfrom, vaddr, to);