rb_set_head_page
struct buffer_page *hpage = rb_set_head_page(cpu_buffer);
iter->head_page = rb_set_head_page(cpu_buffer);
bpage = rb_set_head_page(cpu_buffer);
reader = rb_set_head_page(cpu_buffer);
first_subbuf = subbuf = rb_set_head_page(cpu_buffer);