练习1: 实现 first-fit 连续物理内存分配算法
相关文件
实现 first-fit 连续物理内存分配算法
背景知识
什么是
first-fit 连续物理内存分配算法首次适应(first fit, FF)算法，要求空闲分区链以地址递增的次序链接。在分配内存时，从链首开始顺序查找，直至找到一个大小能满足要求的空闲分区为止。然后再按照作业的大小，从该分区划出一块内存空间，分配给请求者，余下的空闲分区仍留在空闲链中。若从链首直至链尾都不能找到一个能满足要求的分区，则表明系统中已没有足够大的内存分配给该进程，内存分配失败，返回。
first fit算法的特点是什么该算法倾向于优先利用内存中低地址部分的空闲分区，从而保留了高址部分的大空闲区。这为以后到达的大作业分配大的内存空间创造了条件。其缺点是低址部分不断被划分，会留下许多难以利用的、很小的空闲分区，称为碎片。而每次查找又是从低址部分开始的，这无疑又会增加查找可用分区时的开销。
分区分配的操作是什么
分配内存：
系统应利用某种分配算法，从空闲分区链(表)中找到所需大小的分区。设请求的分区大小为
u.size，表中每个空闲分区的大小可表示为m.size，若m.size-u.size<=size(size是事先规定的不再切割的剩余分区的大小)，说明多余部分太小，可不再切割，将整个分区分配给请求者。否则(即多余部分超过size)，便从该分区中按请求的大小划分出一块内存空间分配出去，余下的部分仍留在空闲分区链(表)中。然后，将分配区的首址返回给调用者。回收内存
当进程运行完毕释放内存时，系统根据回收区的首址，从空闲区链(表)中找到相应的插入点，此时可能出现以下四种情况之一：
回收区与插入点的前一个空闲分区F1相邻接。此时应将回收区与插入点的前一分区合并，不必为回收分区分配新表项，而只需修改前一分区F1的大小。
回收分区与插入点的后一空闲分区F2相邻接。此时也可将两份区合并，形成新的空闲分区，但用回收区的首地址作为新空闲区的首址，大小为两者之和。
回收区同时与插入点的前、后两个分区邻接。此时将三个分区合并，使用F1的表项和F1的首址，取消F2的表项，大小为三者之和。
回收区既不与F1邻接，又不与F2邻接。这时应为回收区单独建立一个新表项，填写回收区的首址和大小，并根据其首地址插入到空闲链中的适当位置。
练习内容
结合 清华大学出版社 严蔚敏 《数据结构》，第196~198页，第8.2节，了解First Fit算法，并重写以下函数：default_init，default_init_memmap，default_alloc_pages，default_free_pages。
练习细节
// File: default_pmm.c
// Line: 18-95
/*
* Details of FFMA
* (1) Preparation:
* In order to implement the First-Fit Memory Allocation (FFMA), we should
* manage the free memory blocks using a list. The struct `free_area_t` is used
* for the management of free memory blocks.
* First, you should get familiar with the struct `list` in list.h. Struct
* `list` is a simple doubly linked list implementation. You should know how to
* USE `list_init`, `list_add`(`list_add_after`), `list_add_before`, `list_del`,
* `list_next`, `list_prev`.
* There's a tricky method that is to transform a general `list` struct to a
* special struct (such as struct `page`), using the following MACROs: `le2page`
* (in memlayout.h), (and in future labs: `le2vma` (in vmm.h), `le2proc` (in
* proc.h), etc).
* (2) `default_init`:
* You can reuse the demo `default_init` function to initialize the `free_list`
* and set `nr_free` to 0. `free_list` is used to record the free memory blocks.
* `nr_free` is the total number of the free memory blocks.
* (3) `default_init_memmap`:
* CALL GRAPH: `kern_init` --> `pmm_init` --> `page_init` --> `init_memmap` -->
* `pmm_manager` --> `init_memmap`.
* This function is used to initialize a free block (with parameter `addr_base`,
* `page_number`). In order to initialize a free block, firstly, you should
* initialize each page (defined in memlayout.h) in this free block. This
* procedure includes:
* - Setting the bit `PG_property` of `p->flags`, which means this page is
* valid. P.S. In function `pmm_init` (in pmm.c), the bit `PG_reserved` of
* `p->flags` is already set.
* - If this page is free and is not the first page of a free block,
* `p->property` should be set to 0.
* - If this page is free and is the first page of a free block, `p->property`
* should be set to be the total number of pages in the block.
* - `p->ref` should be 0, because now `p` is free and has no reference.
* After that, We can use `p->page_link` to link this page into `free_list`.
* (e.g.: `list_add_before(&free_list, &(p->page_link));` )
* Finally, we should update the sum of the free memory blocks: `nr_free += n`.
* (4) `default_alloc_pages`:
* Search for the first free block (block size >= n) in the free list and reszie
* the block found, returning the address of this block as the address required by
* `malloc`.
* (4.1)
* So you should search the free list like this:
* list_entry_t le = &free_list;
* while((le=list_next(le)) != &free_list) {
* ...
* (4.1.1)
* In the while loop, get the struct `page` and check if `p->property`
* (recording the num of free pages in this block) >= n.
* struct Page *p = le2page(le, page_link);
* if(p->property >= n){ ...
* (4.1.2)
* If we find this `p`, it means we've found a free block with its size
* >= n, whose first `n` pages can be malloced. Some flag bits of this page
* should be set as the following: `PG_reserved = 1`, `PG_property = 0`.
* Then, unlink the pages from `free_list`.
* (4.1.2.1)
* If `p->property > n`, we should re-calculate number of the rest
* pages of this free block. (e.g.: `le2page(le,page_link))->property
* = p->property - n;`)
* (4.1.3)
* Re-caluclate `nr_free` (number of the the rest of all free block).
* (4.1.4)
* return `p`.
* (4.2)
* If we can not find a free block with its size >=n, then return NULL.
* (5) `default_free_pages`:
* re-link the pages into the free list, and may merge small free blocks into
* the big ones.
* (5.1)
* According to the base address of the withdrawed blocks, search the free
* list for its correct position (with address from low to high), and insert
* the pages. (May use `list_next`, `le2page`, `list_add_before`)
* (5.2)
* Reset the fields of the pages, such as `p->ref` and `p->flags` (PageProperty)
* (5.3)
* Try to merge blocks at lower or higher addresses. Notice: This should
* change some pages' `p->property` correctly.
*/
其大意为：
准备
熟悉
list.h中的结构体list及其接口，并了解le2page等宏的作用；default_init你可以重复使用
default_init函数初始化free_list和nr_free。free_list用来记录空闲的内存块。nr_free是空闲内存块的总数。default_init_memmap调用路径：
kern_init-->pmm_init-->page_init-->init_memmap-->pmm_manager-->init_memmap.该函数被用来初始化一个空闲内存块。
default_alloc_pages在空闲块的链表内搜索第一个空闲的内存块，并重新设置该块的尺寸。
default_free_pages将内存页重新连接到空闲块的链表中，有可能会将几个小的空闲内存块合并为一个大的内存块。
代码解析
default_init在这之前，我们先根据要求，打开
libs/list.h并认真查看。然后，我们再来看一遍注释：// File: default_pmm.c /* * (2) `default_init`: * You can reuse the demo `default_init` function to initialize the `free_list` * and set `nr_free` to 0. `free_list` is used to record the free memory blocks. * `nr_free` is the total number of the free memory blocks. */这里告诉我们，我们需要做的是初始化
free_list，然后将nr_free设为0。可以看到，在该函数之前，曾定义了
free_area与两个相关的宏free_list，nr_free：// File: default_pmm.c // Line: 96-99 free_area_t free_area; #define free_list (free_area.free_list) #define nr_free (free_area.nr_free)其中，
free_area_t的定义为// File: memlayout.h // Line: 155-158 /* free_area_t - maintains a doubly linked list to record free (unused) pages */ typedef struct { list_entry_t free_list; // the list header unsigned int nr_free; // # of free pages in this free list } free_area_t;显然，在
default_pmm.c中，宏free_list和nr_free即为变量free_area的两个成员，这样写调用更为简便。那么，如何对
free_list进行初始化呢？我们把目光转向list.h，发现有这样一段代码：// File: default_pmm.c /* * * list_init - initialize a new entry * @elm: new entry to be initialized * */ static inline void list_init(list_entry_t *elm) { elm->prev = elm->next = elm; }显然，这就是用来初始化
list的。好了，问题解决了，最终的代码如下：// File: default_pmm.c static void default_init(void) { list_init(&free_list); nr_free = 0; }default_init_memmap还是先来看注释：
// File: default_pmm.c /* * (3) `default_init_memmap`: * CALL GRAPH: `kern_init` --> `pmm_init` --> `page_init` --> `init_memmap` --> * `pmm_manager` --> `init_memmap`. * This function is used to initialize a free block (with parameter `addr_base`, * `page_number`). In order to initialize a free block, firstly, you should * initialize each page (defined in memlayout.h) in this free block. This * procedure includes: * - Setting the bit `PG_property` of `p->flags`, which means this page is * valid. P.S. In function `pmm_init` (in pmm.c), the bit `PG_reserved` of * `p->flags` is already set. * - If this page is free and is not the first page of a free block, * `p->property` should be set to 0. * - If this page is free and is the first page of a free block, `p->property` * should be set to be the total number of pages in the block. * - `p->ref` should be 0, because now `p` is free and has no reference. * After that, We can use `p->page_link` to link this page into `free_list`. * (e.g.: `list_add_before(&free_list, &(p->page_link));` ) * Finally, we should update the sum of the free memory blocks: `nr_free += n`. */该函数的功能是初始化一个内存块，
为了初始化一个空闲的内存块，首先你需要初始化该空闲的内存块中每一页。这个过程包括：
设置
p->flags的PG_property位，这意味着这一页是可用的。P.S. 在函数pmm_init（在pmm.c中），PG_reserved位已经被设置好。如果该页是可用的而且不是一个空闲内存块的第一页，那么
p->property应该被设置为0。如果该页是可用的而且是一个空闲内存块的第一页，那么
p->property应该被设置为该块的总页数。p->ref应该被设为0,因为现在p是可用的而且不含有引用。
在这之后，我们可以使用
p->page_link来将该页连接到free_list。（例如：list_add_before(&free_list, &(p->page_link));）最后，我们应该更新空闲内存块的总数：
nr_free += n。以上为注释的含义。那么，在此需要提起的是，在
memlayout.h中定义了两个宏以及对其的一系列操作：// File: memlayout.h // Line: 106-115 /* Flags describing the status of a page frame */ #define PG_reserved 0 // if this bit=1: the Page is reserved for kernel, cannot be used in alloc/free_pages; otherwise, this bit=0 #define PG_property 1 // if this bit=1: the Page is the head page of a free memory block(contains some continuous_addrress pages), and can be used in alloc_pages; if this bit=0: if the Page is the the head page of a free memory block, then this Page and the memory block is alloced. Or this Page isn't the head page. #define SetPageReserved(page) set_bit(PG_reserved, &((page)->flags)) #define ClearPageReserved(page) clear_bit(PG_reserved, &((page)->flags)) #define PageReserved(page) test_bit(PG_reserved, &((page)->flags)) #define SetPageProperty(page) set_bit(PG_property, &((page)->flags)) #define ClearPageProperty(page) clear_bit(PG_property, &((page)->flags)) #define PageProperty(page) test_bit(PG_property, &((page)->flags))这两个宏是内存块所在
page的flag，其中，PG_reserved是flag的第0位，当其值为1时表示该页是内核预留页，不能被alloc或free_pages使用；PG_property是flag的第一位，当其值为1时表示该页是一个空闲内存块的第一个页，可以被alloc_pages使用，当其值为0时，如果此页是一个空闲内存块的第一个页，那么该页以及该内存块已经被分配。以下几个宏(函数)均为对这两个flag的操作，不再做赘述。
那么根据提示，我们很容易就可以写出以下代码：
// File: default_pmm.c // Line: 107-120 static void default_init_memmap(struct Page *base, size_t n) { assert(n > 0); struct Page *p = base; for (; p != base + n; p ++) { assert(PageReserved(p)); p->flags = p->property = 0; set_page_ref(p, 0); } base->property = n; SetPageProperty(base); nr_free += n; list_add(&free_list, &(base->page_link)); }需要说明的是，最后的
list_add过程，注释中只是举了例子，并不是真的要这样写，否则代码会无法正常运行。default_alloc_pages老规矩，先看注释：
// File: default_pmm.c /* * (4) `default_alloc_pages`: * Search for the first free block (block size >= n) in the free list and reszie * the block found, returning the address of this block as the address required by * `malloc`. * (4.1) * So you should search the free list like this: * list_entry_t le = &free_list; * while((le=list_next(le)) != &free_list) { * ... * (4.1.1) * In the while loop, get the struct `page` and check if `p->property` * (recording the num of free pages in this block) >= n. * struct Page *p = le2page(le, page_link); * if(p->property >= n){ ... * (4.1.2) * If we find this `p`, it means we've found a free block with its size * >= n, whose first `n` pages can be malloced. Some flag bits of this page * should be set as the following: `PG_reserved = 1`, `PG_property = 0`. * Then, unlink the pages from `free_list`. * (4.1.2.1) * If `p->property > n`, we should re-calculate number of the rest * pages of this free block. (e.g.: `le2page(le,page_link))->property * = p->property - n;`) * (4.1.3) * Re-caluclate `nr_free` (number of the the rest of all free block). * (4.1.4) * return `p`. * (4.2) * If we can not find a free block with its size >=n, then return NULL. */(4)
default_alloc_pages：在空间的内存链表中搜索第一个可用的内存块（块尺寸>=n）并重新设置发现块的尺寸，然后该块作为被
malloc调用得到的地址被返回。(4.1) 所以你应该在空闲的内存链表中像这样搜索：
list_entry_t le = &free_list; while ((le=list_next(le)) != &free_list) {...}(4.1.1) 在
while循环中，获得结构体page并检查该块所含空闲页数（p->property）是否大于等于nstruct Page *p = le2page(le, page_link); if (p->property >= n) {...}(4.1.2) 如果我们找到这个
p，这就意味着我们找到了一个尺寸大于等于n的空闲内存块，它的前n页可以被申请。该页的一些标志位应该被设置为：PG_reserved = 1，PG_property = 0。然后，取消该页与free_list的链接。- (4.1.2.1) 如果
p->property大于n，我们应该重新计算该内存块的剩余页数。（例如：le2page(le, page_link)->property = p->property - n;）
- (4.1.2.1) 如果
(4.1.3) 重新计算
nr_free（剩余所有空闲内存页的数量）（译者注：原翻译为块）。(4.1.4) 返回
p。
(4.2) 如果我们没有找到尺寸大于等于
n的空闲内存块，那么返回NULL。
据解释可知，该函数作用为分配内存，即在空闲块的链表内搜索第一个空闲的内存块，并重新设置该块的尺寸。结合以上讲解很容易理解，代码也很容易写出：
// File: default_pmm.c // Line: 122-148 static struct Page * default_alloc_pages(size_t n) { assert(n > 0); if (n > nr_free) { return NULL; } struct Page *page = NULL; list_entry_t *le = &free_list; while ((le = list_next(le)) != &free_list) { struct Page *p = le2page(le, page_link); if (p->property >= n) { page = p; break; } } if (page != NULL) { if (page->property > n) { struct Page *p = page + n; p->property = page->property - n; SetPageProperty(p); list_add_after(&(page->page_link), &(p->page_link)); } list_del(&(page->page_link)); nr_free -= n; ClearPageProperty(page); } return page; }default_free_pages// File: default_pmm.c /* * (5) `default_free_pages`: * re-link the pages into the free list, and may merge small free blocks into * the big ones. * (5.1) * According to the base address of the withdrawed blocks, search the free * list for its correct position (with address from low to high), and insert * the pages. (May use `list_next`, `le2page`, `list_add_before`) * (5.2) * Reset the fields of the pages, such as `p->ref` and `p->flags` (PageProperty) * (5.3) * Try to merge blocks at lower or higher addresses. Notice: This should * change some pages' `p->property` correctly. */(5)
default_free_pages：重新将页连接到空闲内存列表中，有可能将一些小的内存块合并为一个大的内存块。
(5.1) 通过独立内存块的
base地址，搜索它正确的位置（从低到高），并插入该页。（可能使用list_next，le2page，list_add_before）(5.2) 重置这些页的参数，例如
p->ref和p->flags（PageProperty）。(5.3) 尝试合并低或高地址的内存块。注意：应该正确修改一些页的
p->property。
这部分代码的顺序可能和注释不太一样，但无伤大雅。难点在于内存块的合并。
提示一下，当合并的时候，对于某一内存块中的第一页
p，当p + p->property == base或者base + base->property == p时就该将这两块合并了。好了，话不多说，上代码。
// File: default_pmm.c static void default_free_pages(struct Page *base, size_t n) { assert(n > 0); struct Page *p = base; for (; p != base + n; p ++) { assert(!PageReserved(p) && !PageProperty(p)); p->flags = 0; set_page_ref(p, 0); } base->property = n; SetPageProperty(base); list_entry_t *le = list_next(&free_list); while (le != &free_list) { p = le2page(le, page_link); le = list_next(le); if (base + base->property == p) { base->property += p->property; ClearPageProperty(p); list_del(&(p->page_link)); } else if (p + p->property == base) { p->property += base->property; ClearPageProperty(base); base = p; list_del(&(p->page_link)); } } nr_free += n; list_add(&free_list, &(base->page_link)); }
参考文献
数据结构（C语言版），严蔚敏，清华大学出版社
计算机操作系统（第四版），汤小丹，西安电子科技大学出版社