从fat看文件系统的加载流程走读

本方主要从以下几面方面来研究fs文件系统

1、文件系统的注册
2、文件系统的mount
3、文件的访问

一、 fat文件系统的注册

static struct file_system_type vfat_fs_type = {.owner		= THIS_MODULE,.name		= "vfat",.mount		= vfat_mount,.kill_sb	= kill_block_super,.fs_flags	= FS_REQUIRES_DEV | FS_ALLOW_IDMAP,
};
MODULE_ALIAS_FS("vfat");static int __init init_vfat_fs(void)
{return register_filesystem(&vfat_fs_type);
}int register_filesystem(struct file_system_type * fs)
{int res = 0;struct file_system_type ** p;if (fs->parameters &&!fs_validate_description(fs->name, fs->parameters))return -EINVAL;BUG_ON(strchr(fs->name, '.'));if (fs->next)return -EBUSY;write_lock(&file_systems_lock);p = find_filesystem(fs->name, strlen(fs->name));//从已经注册的全局变量中依次轮循进行字符串比较if (*p) //不为空则证明已经注册过res = -EBUSY;else //为空则直接用结构体变量填充*p = fs;write_unlock(&file_systems_lock);return res;
}static struct file_system_type **find_filesystem(const char *name, unsigned len)
{struct file_system_type **p;//从全局变量file_systems查找，注意file_systems默认值是NULL，返回的是指针地址，用以接收需要注册的结构体数据for (p = &file_systems; *p; p = &(*p)->next)if (strncmp((*p)->name, name, len) == 0 &&!(*p)->name[len])break;return p;
}

以上可知，文件系统注册是比较简单的单链表操作

文件系统的挂载

挂载需要上层主动调用mount命令，通过系统调用到内核进行挂载。以fat文件格式为例看挂载流程

long do_mount(const char *dev_name, const char __user *dir_name,const char *type_page, unsigned long flags, void *data_page)
{struct path path;int ret;ret = user_path_at(AT_FDCWD, dir_name, LOOKUP_FOLLOW, &path); //用当前线程task成员path填充path结构体if (ret)return ret;ret = path_mount(dev_name, &path, type_page, flags, data_page);path_put(&path);return ret;
}

static int do_new_mount(struct path *path, const char *fstype, int sb_flags,int mnt_flags, const char *name, void *data)
{struct file_system_type *type;struct fs_context *fc;const char *subtype = NULL;int err = 0;if (!fstype)return -EINVAL;type = get_fs_type(fstype);//根据字符串从全局链表上获取注册的文件数据if (!type)return -ENODEV;if (type->fs_flags & FS_HAS_SUBTYPE) {subtype = strchr(fstype, '.');if (subtype) {subtype++;if (!*subtype) {put_filesystem(type);return -EINVAL;}}}fc = fs_context_for_mount(type, sb_flags);//分配fs_context结构体，比较重要put_filesystem(type);if (IS_ERR(fc))return PTR_ERR(fc);if (subtype)err = vfs_parse_fs_string(fc, "subtype",subtype, strlen(subtype));if (!err && name)err = vfs_parse_fs_string(fc, "source", name, strlen(name));if (!err)err = parse_monolithic_mount_data(fc, data);if (!err && !mount_capable(fc))err = -EPERM;if (!err)err = vfs_get_tree(fc);//回调函数到具体的文件系统里面，创建超级块if (!err)err = do_new_mount_fc(fc, path, mnt_flags);//创建挂载点put_fs_context(fc);return err;
}

fs_context_for_mount
->alloc_fs_context

static struct fs_context *alloc_fs_context(struct file_system_type *fs_type,struct dentry *reference,unsigned int sb_flags,unsigned int sb_flags_mask,enum fs_context_purpose purpose)
{int (*init_fs_context)(struct fs_context *);struct fs_context *fc;int ret = -ENOMEM;fc = kzalloc(sizeof(struct fs_context), GFP_KERNEL_ACCOUNT);if (!fc)return ERR_PTR(-ENOMEM);fc->purpose	= purpose;fc->sb_flags	= sb_flags;fc->sb_flags_mask = sb_flags_mask;fc->fs_type	= get_filesystem(fs_type);//注册的文件系统填充，后续会被使用fc->cred	= get_current_cred();fc->net_ns	= get_net(current->nsproxy->net_ns);fc->log.prefix	= fs_type->name;mutex_init(&fc->uapi_mutex);switch (purpose) {case FS_CONTEXT_FOR_MOUNT:fc->user_ns = get_user_ns(fc->cred->user_ns);break;case FS_CONTEXT_FOR_SUBMOUNT:fc->user_ns = get_user_ns(reference->d_sb->s_user_ns);break;case FS_CONTEXT_FOR_RECONFIGURE:atomic_inc(&reference->d_sb->s_active);fc->user_ns = get_user_ns(reference->d_sb->s_user_ns);fc->root = dget(reference);break;}/* TODO: Make all filesystems support this unconditionally */init_fs_context = fc->fs_type->init_fs_context;//函数指向文件结构体的init_fs_context成员函数if (!init_fs_context)init_fs_context = legacy_init_fs_context;//如果文件格式没有定义此函数则使用默认函数。对于fat文件格式来说是没有定义的，ext4有定义ret = init_fs_context(fc);if (ret < 0)goto err_fc;fc->need_free = true;return fc;
}
static int legacy_init_fs_context(struct fs_context *fc)
{fc->fs_private = kzalloc(sizeof(struct legacy_fs_context), GFP_KERNEL_ACCOUNT);if (!fc->fs_private)return -ENOMEM;fc->ops = &legacy_fs_context_ops;//设置其默认的回调函数，后续这个ops会被使用到 return 0;
}

vfs_get_tree
->fc->ops->get_tree //这里就会使用到上述的legacy_fs_context_ops

const struct fs_context_operations legacy_fs_context_ops = {.free			= legacy_fs_context_free,.dup			= legacy_fs_context_dup,.parse_param		= legacy_parse_param,.parse_monolithic	= legacy_parse_monolithic,.get_tree		= legacy_get_tree,.reconfigure		= legacy_reconfigure,
};static int legacy_get_tree(struct fs_context *fc)
{struct legacy_fs_context *ctx = fc->fs_private;struct super_block *sb;struct dentry *root;root = fc->fs_type->mount(fc->fs_type, fc->sb_flags,fc->source, ctx->legacy_data);//调用文件系统的mount函数，如果没有定义init_fs_context这个回调函数则mount函数一定要存在不然直接crash.反之则不一定需要mountif (IS_ERR(root))return PTR_ERR(root);sb = root->d_sb;BUG_ON(!sb);fc->root = root; //保存rootreturn 0;
}

接下来就是调用 fat文件系统提供的mount函数了，看下大体流程

上述的流程核心就是创建super_block,并各种初始化填充里面的数据成员，另外一个就是生成root inode。函数比较复杂这里不作深入研究，只看大体的流程。在这里还没有走到对inode节点文件操作回调赋值。
do_new_mount_fc创建一个新的挂载点，这里就详细研究了

3、文件的访问

分成两部分：
一、目录的创建
二、文件打开

目录创建由用户调用命令mkdir通过系统调用到驱动，大体流程如下

mkdir核心内容是创建一个inode并加入到链表，这个inode节点会被赋值对文件和目录操作的回调函数，这里只看下核心代码

int fat_fill_inode(struct inode *inode, struct msdos_dir_entry *de)
{struct msdos_sb_info *sbi = MSDOS_SB(inode->i_sb);int error;MSDOS_I(inode)->i_pos = 0;inode->i_uid = sbi->options.fs_uid;inode->i_gid = sbi->options.fs_gid;inode_inc_iversion(inode);inode->i_generation = get_random_u32();if ((de->attr & ATTR_DIR) && !IS_FREE(de->name)) {inode->i_generation &= ~1;inode->i_mode = fat_make_mode(sbi, de->attr, S_IRWXUGO);inode->i_op = sbi->dir_ops;inode->i_fop = &fat_dir_operations;//目录的操作回调函数MSDOS_I(inode)->i_start = fat_get_start(sbi, de);MSDOS_I(inode)->i_logstart = MSDOS_I(inode)->i_start;error = fat_calc_dir_size(inode);if (error < 0)return error;MSDOS_I(inode)->mmu_private = inode->i_size;set_nlink(inode, fat_subdirs(inode));error = fat_validate_dir(inode);if (error < 0)return error;} else { /* not a directory */inode->i_generation |= 1;inode->i_mode = fat_make_mode(sbi, de->attr,((sbi->options.showexec && !is_exec(de->name + 8))? S_IRUGO|S_IWUGO : S_IRWXUGO));MSDOS_I(inode)->i_start = fat_get_start(sbi, de);MSDOS_I(inode)->i_logstart = MSDOS_I(inode)->i_start;inode->i_size = le32_to_cpu(de->size);inode->i_op = &fat_file_inode_operations;inode->i_fop = &fat_file_operations;//文件的操作回调函数inode->i_mapping->a_ops = &fat_aops;MSDOS_I(inode)->mmu_private = inode->i_size;}if (de->attr & ATTR_SYS) {if (sbi->options.sys_immutable)inode->i_flags |= S_IMMUTABLE;}fat_save_attrs(inode, de->attr);inode->i_blocks = ((inode->i_size + (sbi->cluster_size - 1))& ~((loff_t)sbi->cluster_size - 1)) >> 9;fat_time_fat2unix(sbi, &inode->i_mtime, de->time, de->date, 0);inode->i_ctime = inode->i_mtime;if (sbi->options.isvfat) {fat_time_fat2unix(sbi, &inode->i_atime, 0, de->adate, 0);fat_time_fat2unix(sbi, &MSDOS_I(inode)->i_crtime, de->ctime,de->cdate, de->ctime_cs);} elseinode->i_atime = fat_truncate_atime(sbi, &inode->i_mtime);return 0;
}

这个是对目录的操作，文件操作也会走到上面的这个函数拿到inode节点并赋值回调函数
文件打开的的基本流程如下：

基本操作也是拿到inode，后续通过inode进行操作。如果有实现open回调函数则调用。但是fat好像并没有实现这个回调

	f->f_op = fops_get(inode->i_fop);if (WARN_ON(!f->f_op)) {error = -ENODEV;goto cleanup_all;}error = security_file_open(f);if (error)goto cleanup_all;error = break_lease(file_inode(f), f->f_flags);if (error)goto cleanup_all;/* normally all 3 are set; ->open() can clear them if needed */f->f_mode |= FMODE_LSEEK | FMODE_PREAD | FMODE_PWRITE;if (!open)open = f->f_op->open;if (open) {error = open(inode, f);if (error)goto cleanup_all;}

最后打开文件成功后将文件保存在fdtable数组里面