本文主要完成MIT 6.S081 实验八：file system
开始之前，切换分支：
$ git fetch
$ git checkout fs
$ make clean
The format of an on-disk
inodeis defined bystruct dinodeinfs.h. You’re particularly interested in NDIRECT, NINDIRECT, MAXFILE, and the addrs[] element ofstruct dinode. The code that finds a file’s data on disk is inbmap()infs.c. Have a look at it and make sure you understand what it’s doing.bmap()is called both when reading and writing a file. When writing,bmap()allocates new blocks as needed to hold file content, as well as allocating an indirect block if needed to hold block addresses.
bmap()deals with two kinds of block numbers. The bn argument is a “logical block number” – a block number within the file, relative to the start of the file. The block numbers inip->addrs[], and the argument tobread(), are disk block numbers. You can viewbmap()as mapping a file’s logical block numbers into disk block numbers.
Modify
bmap()so that it implements a doubly-indirect block, in addition to direct blocks and a singly-indirect block. You’ll have to have only 11 direct blocks, rather than 12, to make room for your new doubly-indirect block; you’re not allowed to change the size of an on-disk inode. The first 11 elements ofip->addrs[]should be direct blocks; the 12th should be a singly-indirect block (just like the current one); the 13th should be your new doubly-indirect block.
- Make sure you understand
bmap(). Write out a diagram of the relationships betweenip->addrs[], the indirect block, the doubly-indirect block and the singly-indirect blocks it points to, and data blocks. Make sure you understand why adding a doubly-indirect block increases the maximum file size by256*256blocks (really -1, since you have to decrease the number of direct blocks by one).- Think about how you’ll index the doubly-indirect block, and the indirect blocks it points to, with the logical block number.
- If you change the definition of NDIRECT, you’ll probably have to change the declaration of
addrs[]instruct inodeinfile.h. Make sure thatstruct inodeandstruct dinodehave the same number of elements in theiraddrs[]arrays.- If you change the definition of NDIRECT, make sure to create a new
fs.img, since mkfs uses NDIRECT to build the file system.
If your file system gets into a bad state, perhaps by crashing, delete fs.img (do this from Unix, not xv6). make will build a new clean file system image for you.- Don’t forget to
brelse()each block that youbread().- You should allocate indirect blocks and doubly-indirect blocks only as needed, like the original
bmap().- Make sure itrunc frees all blocks of a file, including double-indirect blocks.
上面说得很清楚，xv6 不支持太大的单文件，修改 bmap() 使其能用一个 double-indirect block，从而映射更多的块（256*256+256+11=65803）。
首先看看 (d)inode 的主要字段：
#define NDIRECT 12
// in-memory copy of an inode
struct inode {
...
short type; // copy of disk inode
short major;
short minor;
short nlink;
uint size;
uint addrs[NDIRECT+1];
};
可以看到，addrs[] 字段有 13 个空间，我们要用前 11 个直接映射，第 12 个一级间接映射，第 13 个二级间接映射。要做这些，就得修改 bmap()：
// Inode content
//
// The content (data) associated with each inode is stored
// in blocks on the disk. The first NDIRECT block numbers
// are listed in ip->addrs[]. The next NINDIRECT blocks are
// listed in block ip->addrs[NDIRECT].
// Return the disk block address of the nth block in inode ip.
// If there is no such block, bmap allocates one.
// returns 0 if out of disk space.
static uint
bmap(struct inode *ip, uint bn)
{
uint addr, *a;
struct buf *bp;
if(bn < NDIRECT){
if((addr = ip->addrs[bn]) == 0){
addr = balloc(ip->dev);
if(addr == 0)
return 0;
ip->addrs[bn] = addr;
}
return addr;
}
bn -= NDIRECT;
if(bn < NINDIRECT){
// Load indirect block, allocating if necessary.
if((addr = ip->addrs[NDIRECT]) == 0){
addr = balloc(ip->dev);
if(addr == 0)
return 0;
ip->addrs[NDIRECT] = addr;
}
bp = bread(ip->dev, addr);
a = (uint*)bp->data;
if((addr = a[bn]) == 0){
addr = balloc(ip->dev);
if(addr){
a[bn] = addr;
log_write(bp);
}
}
brelse(bp);
return addr;
}
panic("bmap: out of range");
}
bmap() 函数根据 inode 以及 logic block number 返回物理磁盘块号。如果没有这个磁盘块号，那就给它分配一个，赋值之后返回。bmap()目前只能映射逻辑块号为 0——11（直接映射）、12——267（间接映射）的物理块号。
我们需要在里面设计二级间接映射，并且重新设计 逻辑块号0——10（直接映射）、11——266（一级间接映射）、267——65802（二级间接映射）的物理块号映射。
以下是注释以及部分代码：
// 直接映射
// 一级间接映射
// 二级间接映射
// 逻辑块号计算第一个块号（0~255）
// 继续计算第二个 块号（0~255）
首先修改宏定义：
#define NDIRECT 11
#define NINDIRECT (BSIZE / sizeof(uint))
#define MAXFILE (NDIRECT + NINDIRECT + NINDIRECT * NINDIRECT)
修改后的代码如下：
static uint
bmap(struct inode *ip, uint bn)
{
uint addr, *a;
struct buf *bp;
// 0~10(11个) 直接映射
if(bn < NDIRECT){
if((addr = ip->addrs[bn]) == 0){
addr = balloc(ip->dev);
if(addr == 0)
return 0;
ip->addrs[bn] = addr;
}
return addr;
}
bn = bn - NDIRECT;
// 11~256+11(267 个) 一级间接映射
if(bn < NINDIRECT){
// Load indirect block, allocating if necessary.
if((addr = ip->addrs[NDIRECT]) == 0){
addr = balloc(ip->dev);
if(addr == 0)
return 0;
ip->addrs[NDIRECT] = addr;
}
bp = bread(ip->dev, addr);
a = (uint*)bp->data;
if((addr = a[bn]) == 0){
addr = balloc(ip->dev);
if(addr){
a[bn] = addr;
log_write(bp);
}
}
brelse(bp);
return addr;
}
bn = bn - NINDIRECT;
// 267~256*256+267(65803 个) 二级间接映射
if(bn < NINDIRECT * NINDIRECT){
// Load double-indirect block, allocating if necessary.
// 拿到这个块号
uint bn1 = bn/256;
uint bn2 = bn%256;
if((addr = ip->addrs[NDIRECT+1]) == 0){
addr = balloc(ip->dev);
if(addr == 0)
return 0;
ip->addrs[NDIRECT+1] = addr;
}
// 拿到一级块
bp = bread(ip->dev, addr);
a = (uint*)bp->data;
if((addr = a[bn1]) == 0){
addr = balloc(ip->dev);
if(addr){
a[bn1] = addr;
log_write(bp);
}
}
brelse(bp);
// 拿到二级块
bp = bread(ip->dev, addr);
a = (uint*)bp->data;
if((addr = a[bn2]) == 0){
addr = balloc(ip->dev);
if(addr){
a[bn2] = addr;
log_write(bp);
}
}
brelse(bp);
return addr;
}
panic("bmap: out of range");
}
当然，修改好了以后，还要考虑释放块：
// Truncate inode (discard contents).
// Caller must hold ip->lock.
void
itrunc(struct inode *ip)
{
...
// 释放二级间接
if(ip->addrs[NDIRECT+1]){
bp = bread(ip->dev, ip->addrs[NDIRECT+1]);
a = (uint*)bp->data;
struct buf *bp2;
uint *a1;
for(j = 0; j < NINDIRECT; j++){
if(a[j]){
bp2 = bread(ip->dev, a[j]);
a1 = (uint*)bp2->data;
for(int k = 0; k < NINDIRECT; k++){
if(a1[k]){
bfree(ip->dev, a1[k]);
}
}
brelse(bp2);
// 释放二级间接块
bfree(ip->dev, a[j]);
}
}
brelse(bp);
bfree(ip->dev, ip->addrs[NDIRECT+1]);
ip->addrs[NDIRECT+1] = 0;
}
ip->size = 0;
iupdate(ip);
}
这样，这个 lab 就完成了！（没有参考任何代码，完全自己写，一次跑过）：
xv6 kernel is booting
init: starting sh
$ bigfile
.....................
...
wrote 65803 blocks
bigfile done; ok
In this exercise you will add symbolic links to xv6. Symbolic links (or soft links) refer to a linked file by pathname; when a symbolic link is opened, the kernel follows the link to the referred file. Symbolic links resembles hard links, but hard links are restricted to pointing to file on the same disk, while symbolic links can cross disk devices. Although xv6 doesn’t support multiple devices, implementing this system call is a good exercise to understand how pathname lookup works.
You will implement the
symlink(char *target, char *path)system call, which creates a new symbolic link at path that refers to file named by target. For further information, see the man page symlink. To test, add symlinktest to the Makefile and run it.
- First, create a new system call number for symlink, add an entry to
user/usys.pl,user/user.h, and implement an empty sys_symlink inkernel/sysfile.c.- Add a new file type (T_SYMLINK) to
kernel/stat.hto represent a symbolic link.
Add a new flag tokernel/fcntl.h, (O_NOFOLLOW), that can be used with the open system call. Note that flags passed to open are combined using a bitwise OR operator, so your new flag should not overlap with any existing flags. This will let you compileuser/symlinktest.conce you add it to the Makefile.- Implement the
symlink(target, path)system call to create a new symbolic link at path that refers to target. Note that target does not need to exist for the system call to succeed. You will need to choose somewhere to store the target path of a symbolic link, for example, in the inode’s data blocks. symlink should return an integer representing success (0) or failure (-1) similar to link and unlink.- Modify the open system call to handle the case where the path refers to a symbolic link. If the file does not exist, open must fail. When a process specifies O_NOFOLLOW in the flags to open, open should open the symlink (and not follow the symbolic link).
- If the linked file is also a symbolic link, you must recursively follow it until a non-link file is reached. If the links form a cycle, you must return an error code. You may approximate this by returning an error code if the depth of links reaches some threshold (e.g., 10).
- Other system calls (e.g., link and unlink) must not follow symbolic links; these system calls operate on the symbolic link itself. You do not have to handle symbolic links to directories for this lab.
要简单理解这个 lab，可以用 Windows OS 中创建快捷方式的行为来类比，链接（软连接和硬链接）是躺在父文件夹中的一个文件。我们的目标就是创建一个文件，然后当用户打开这个文件的时候，返回 fd。
因此，我们要做的事情是两件：
在 xv6 中，创建文件用的是：static struct inode* create(char *path, short type, short major, short minor)：
static struct inode*
create(char *path, short type, short major, short minor)
{
struct inode *ip, *dp;
char name[DIRSIZ];
if((dp = nameiparent(path, name)) == 0)
return 0;
ilock(dp);
if((ip = dirlookup(dp, name, 0)) != 0){
iunlockput(dp);
ilock(ip);
if(type == T_FILE && (ip->type == T_FILE || ip->type == T_DEVICE))
return ip;
iunlockput(ip);
return 0;
}
if((ip = ialloc(dp->dev, type)) == 0){
iunlockput(dp);
return 0;
}
ilock(ip);
ip->major = major;
ip->minor = minor;
ip->nlink = 1;
iupdate(ip);
if(type == T_DIR){ // Create . and .. entries.
// No ip->nlink++ for ".": avoid cyclic ref count.
if(dirlink(ip, ".", ip->inum) < 0 || dirlink(ip, "..", dp->inum) < 0)
goto fail;
}
if(dirlink(dp, name, ip->inum) < 0)
goto fail;
if(type == T_DIR){
// now that success is guaranteed:
dp->nlink++; // for ".."
iupdate(dp);
}
iunlockput(dp);
return ip;
fail:
// something went wrong. de-allocate ip.
ip->nlink = 0;
iupdate(ip);
iunlockput(ip);
iunlockput(dp);
return 0;
}
这个 create 函数是用于在文件系统中创建文件或目录的。它的主要功能包括（大模型解析）：
获取父目录 dp 和要创建的文件或目录名 name：
nameiparent 函数获取给定路径 path 的父目录 inode dp 和要创建的文件或目录名 name。获取父目录的锁 ilock：
dp 进行加锁操作，确保其他进程不会同时修改该目录。检查是否已存在同名文件或目录：
dirlookup 函数检查在父目录 dp 中是否已存在同名的文件或目录。ip。ip 和父目录 dp 的锁，并返回失败（返回 0）。分配新的 inode：
ialloc 函数分配一个新的 inode ip。ip 进行加锁操作。设置新节点的属性：
ip 的类型、主设备号、次设备号、链接计数等属性，并将这些信息写入磁盘（iupdate(ip)）。如果是目录类型 T_DIR：
. 和 .. 条目。dp 的链接计数。dp。连接新节点到父目录：
ip 与父目录 dp 进行连接，创建文件或目录的实际条目。ip。成功创建文件或目录：
dp 的链接计数。dp。失败处理：
ip，将其链接计数设为 0，并更新其信息到磁盘。ip 和父目录 dp。举个例子，要创建一个文件，必须传入一个如/a/b/c的 path，nameiparent(path, name))会返回文件 b 的 inode dp。之后，会在 dp 中检查要被创建的文件 c 是否存在。如果存在直接返回这个文件 c 的 inode，如果不存在，就在 inode block 中分配一个 inode，然后对 inode 做一些设置，之后和父目录关联起来dirlink(dp, name, ip->inum)。这个文件创建完成了，至于 c 是文件夹还是文件，这对于父目录完全透明。也可以看到，创建文件事实上就是在创建 inode。由于目录也是一个文件，链接到父目录的本质是：
// Write a new directory entry (name, inum) into the directory dp.
// Returns 0 on success, -1 on failure (e.g. out of disk blocks).
int
dirlink(struct inode *dp, char *name, uint inum)
{
int off;
struct dirent de;
struct inode *ip;
// Check that name is not present.
if((ip = dirlookup(dp, name, 0)) != 0){
iput(ip);
return -1;
}
// Look for an empty dirent.
for(off = 0; off < dp->size; off += sizeof(de)){
if(readi(dp, 0, (uint64)&de, off, sizeof(de)) != sizeof(de))
panic("dirlink read");
if(de.inum == 0)
break;
}
strncpy(de.name, name, DIRSIZ);
de.inum = inum;
if(writei(dp, 0, (uint64)&de, off, sizeof(de)) != sizeof(de))
return -1;
return 0;
}
本质就是往 inode 绑定的的文件中写入一个 entry，也就是往父目录 b 中写一个 entry。而一个 entry 就是：
struct dirent {
ushort inum;
char name[DIRSIZ];
};
它由 inode 编号和文件名构成。
接下来看看如何往一个文件中写内容，事实上，关于如何向一个文件中写内容，主要是指往这个文件的 inode 中的 addrs 字段指的 block 中写内容。由 writei() 函数实现：
// Write data to inode.
// Caller must hold ip->lock.
// If user_src==1, then src is a user virtual address;
// otherwise, src is a kernel address.
// Returns the number of bytes successfully written.
// If the return value is less than the requested n,
// there was an error of some kind.
int
writei(struct inode *ip, int user_src, uint64 src, uint off, uint n)
{
uint tot, m;
struct buf *bp;
if(off > ip->size || off + n < off)
return -1;
if(off + n > MAXFILE*BSIZE)
return -1;
for(tot=0; tot<n; tot+=m, off+=m, src+=m){
uint addr = bmap(ip, off/BSIZE);
if(addr == 0)
break;
bp = bread(ip->dev, addr);
m = min(n - tot, BSIZE - off%BSIZE);
if(either_copyin(bp->data + (off % BSIZE), user_src, src, m) == -1) {
brelse(bp);
break;
}
log_write(bp);
brelse(bp);
}
if(off > ip->size)
ip->size = off;
// write the i-node back to disk even if the size didn't change
// because the loop above might have called bmap() and added a new
// block to ip->addrs[].
iupdate(ip);
return tot;
}
这个函数 writei 用于向一个 inode 写入数据。它的功能包括（大模型解析）：
参数解释：
struct inode *ip 是要写入数据的目标 inode。int user_src 表示源地址 src 是用户空间的虚拟地址（user virtual address）还是内核地址。uint64 src 是数据的源地址。uint off 是写入数据的偏移量。uint n 是要写入的字节数。参数检查：
off 是否超出了文件大小或范围。如果超出了文件大小或范围，返回错误 -1。-1。循环写入数据：
tot 表示已经成功写入的字节数。bmap 函数获取要写入数据的逻辑块的磁盘块地址。0，表示无法分配新的磁盘块，退出循环。bread 函数读取磁盘块到缓冲区 bp 中。m，考虑到当前磁盘块的可用空间和写入偏移量。either_copyin 函数将数据从源地址 src 复制到缓冲区 bp 中。如果复制失败，释放缓冲区并退出循环。bp 记录到日志（log_write）并释放缓冲区。更新文件大小：
off。更新 inode 到磁盘：
bmap() 并添加了新的块到 ip->addrs[]。返回写入的总字节数：
tot。这个函数其实就是往 inode 编号为 ip 的文件中写数据。
在创建一个文件后，如果这个文件是一个链接文件（比如快捷方式），那么拿到这个文件的 inode 之后，就会往这个文件中写一个很重要的信息，那就是这个链接文件要连接到的 path。比如文件 c 要指向的文件是/a/b/c/d，那么就需要将 /a/b/c/d 写入到这个文件c中。
以上两个函数相当重要，它是创建链接文件的重要函数，必须理解透彻。
根据 hints，我们要实现的 sys_symlink()，就是在创建链接，我们可以在这里面实现创建链接（文件）。代码和注释如下：
// 获取目标链接和链接 path（包含文件名）
// 创建一个文件
// 在文件中写入目标链接
// 文件创建完成
先在 kernel/stat.h 定义必要的文件类型：
#define T_DIR 1 // Directory
#define T_FILE 2 // File
#define T_DEVICE 3 // Device
#defien T_SYMLINK 4 // Symlink
在 kernel/fcntl.h 中定义文件控制宏：
#define O_RDONLY 0x000
#define O_WRONLY 0x001
#define O_RDWR 0x002
#define O_CREATE 0x200
#define O_TRUNC 0x400
#define O_NOFOLLOW 0x800
以下是完整的代码：
uint64
sys_symlink(void){
struct inode *ip;
// 获取目标链接和链接 path（包含文件名）
char target[MAXPATH], path[MAXPATH];
if(argstr(0, target, MAXPATH) < 0 || argstr(1, path, MAXPATH) < 0)
return -1;
begin_op();
// 创建一个文件
ip = create(path, T_SYMLINK, 0, 0);
if(ip == 0){
end_op();
return -1;
}
// 在文件中写入目标链接
if(writei(ip, 0, (uint64)target, 0, strlen(target)) < 0) {
end_op();
return -1;
}
iunlockput(ip);
end_op();
return 0;
}
创建好链接文件后，打开这个文件的时候，需要做出一些判断。当打开的是一个链接文件时，还要判断这个链接文件是否也同样也链接到了一个文件。我们设置一些标识，跟随符号链接，直到跟随到非符号链接的 inode 为止。
现对 sys_open() 函数做出一些修改：
uint64
sys_open(void)
{
...
if(omode & O_CREATE){
ip = create(path, T_FILE, 0, 0);
if(ip == 0){
end_op();
return -1;
}
} else {
// 如果不是创建文件,那么就在这里进行检查
int symlink_depth = 0;
while(1) {
if((ip = namei(path)) == 0){
end_op();
return -1;
}
ilock(ip);
// 如果是链接类型且指明要跟随,意味着要打开它，取出它的链接，进行迭代
if(ip->type == T_SYMLINK && (omode & O_NOFOLLOW) == 0) {
if(++symlink_depth > 10) {
// too many layer of symlinks, might be a loop
iunlockput(ip);
end_op();
return -1;
}
// 读到 path 中
if(readi(ip, 0, (uint64)path, 0, MAXPATH) < 0) {
iunlockput(ip);
end_op();
return -1;
}
iunlockput(ip);
} else {
// 说明这是一个普通文件,退出之后，准备返回
break;
}
}
...
iunlock(ip);
end_op();
return fd;
}
这样，我们就完成了整个实验：
== Test running bigfile ==
$ make qemu-gdb
running bigfile: OK (171.3s)
== Test running symlinktest ==
$ make qemu-gdb
(1.0s)
== Test symlinktest: symlinks ==
symlinktest: symlinks: OK
== Test symlinktest: concurrent symlinks ==
symlinktest: concurrent symlinks: OK
全文完，感谢阅读。