# 练习3

## 练习3：释放某虚地址所在的页并取消对应二级页表项的映射

[练习文档](https://chyyuu.gitbooks.io/ucore_os_docs/content/lab2/lab2_3_2_1_phymemlab_exercise.html)

### 相关文件

* [pmm.c](/ucore-analysis/ucore/kern/mm/pmm_c.md)

## 释放某虚地址所在的页并取消对应二级页表项的映射

### 练习内容

当释放一个包含某虚地址的物理内存页时，需要让对应此物理内存页的管理数据结构 `Page` 做相关的清除处理，使得此物理内存页成为空闲；另外还需把表示虚地址与物理地址对应关系的二级页表项清除。请仔细查看和理解 `page_remove_pte` 函数中的注释。为此，需要补全在 `kern/mm/pmm.c` 中的 `page_remove_pte` 函数。

### 练习细节

按照惯例，先来看一下需要编写的 `page_remove_pte` 的注释：

```c
//    File:    pmm.c

//page_remove_pte - free an Page sturct which is related linear address la
//                - and clean(invalidate) pte which is related linear address la
//note: PT is changed, so the TLB need to be invalidate 
static inline void
page_remove_pte(pde_t *pgdir, uintptr_t la, pte_t *ptep) {
    /* LAB2 EXERCISE 3: YOUR CODE
     *
     * Please check if ptep is valid, and tlb must be manually updated if mapping is updated
     *
     * Maybe you want help comment, BELOW comments can help you finish the code
     *
     * Some Useful MACROs and DEFINEs, you can use them in below implementation.
     * MACROs or Functions:
     *   struct Page *page pte2page(*ptep): get the according page from the value of a ptep
     *   free_page : free a page
     *   page_ref_dec(page) : decrease page->ref. NOTICE: ff page->ref == 0 , then this page should be free.
     *   tlb_invalidate(pde_t *pgdir, uintptr_t la) : Invalidate a TLB entry, but only if the page tables being
     *                        edited are the ones currently in use by the processor.
     * DEFINEs:
     *   PTE_P           0x001                   // page table/directory entry flags bit : Present
     */
#if 0
    if (0) {                      //(1) check if this page table entry is present
        struct Page *page = NULL; //(2) find corresponding page to pte
                                  //(3) decrease page reference
                                  //(4) and free this page when page reference reachs 0
                                  //(5) clear second page table entry
                                  //(6) flush tlb
    }
#endif
}
```

首先，这里说明了函数的作用，即释放一个与线性地址 `la` 相关联的 `Page struct`，并且清理（使无效）与线性地址 `la`相关的 pte。

注意：PT 被改变，所以 TLB 需要被无效化。

接下来结合注释跟代码的提示就很容易写出啦，比如 (1) 就是检查标志位，(2) 调用 `pte2page`，(3) 使用 `page_ref_dec` 减少 `Page` 的引用计数等等。

参考答案：

```c
static inline void
page_remove_pte(pde_t *pgdir, uintptr_t la, pte_t *ptep) {
    if (*ptep & PTE_P)
    {
        struct Page * page = pte2page(*ptep);
        if (page_ref_dec(page) == 0)
            free_page(page);
        *ptep = 0;
        tlb_invalidate(pgdir, la);
    }
}
```

### 问题分析

* 数据结构Page的全局变量（其实是一个数组）的每一项与页表中的页目录项和页表项有无对应关系？如果有，其对应关系是啥？

  首先来看一下全局变量 `pages` 的定义：

  ```c
  // virtual address of physicall page array
  struct Page *pages;
  ```

  还有结构体 `Page` 的定义：

  ```c
  /* *
   * struct Page - Page descriptor structures. Each Page describes one
   * physical page. In kern/mm/pmm.h, you can find lots of useful functions
   * that convert Page to other data types, such as phyical address.
   * */
  struct Page {
      int ref;                        // page frame's reference counter
      uint32_t flags;                 // array of flags that describe the status of the page frame
      unsigned int property;          // the num of free block, used in first fit pm manager
      list_entry_t page_link;         // free list link
  };
  ```

  再来复习一下上一个练习中的知识：线性地址 `la` 是由页目录项 PDE、页表项 PTE、页偏移 PGOFF 组成的。

  结合以上知识，就很容易想到，对于每个有效的页目录项或者页表项，都有一个 `page` 都记录着其信息。
* 如果希望虚拟地址与物理地址相等，则需要如何修改lab2，完成此事？ 鼓励通过编程来具体完成这个问题

  结合参考文献，将虚拟地址与物理地址的偏移改为 `0` 即可。

### 参考文献

* [系统执行中地址映射的三个阶段](https://chyyuu.gitbooks.io/ucore_os_docs/content/lab2/lab2_3_3_5_4_maping_relations.html)


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://oscourse-tsinghua.gitbook.io/ucore-analysis/labs/lab2/practice3.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.