} else {

                     if (next->active_mm != mm) BUG();

                     switch_mm(oldmm, mm, next, this_cpu);       //如果是用户进程，切换页表

              }

              if (!prev->mm) {

                     prev->active_mm = NULL;

                     mmdrop(oldmm);

              }

       }

       /*

        * This just switches the register state and the stack.

        */

       switch_to(prev, next, prev);

       __schedule_tail(prev);

same_process:

       reacquire_kernel_lock(current);

       if (current->need_resched)

              goto need_resched_back;

       return;

}

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