VFS 抽象层 API
1. 模块架构
1.1 功能概述
VFS 通过六种主要 *_operations 函数表结构为具体文件系统提供统一接口。
1.2 关键源文件
| 文件 | 作用 |
|---|---|
include/linux/fs.h | 核心操作定义 |
include/linux/fs/super_types.h | super_operations |
include/linux/dcache.h | dentry_operations |
include/linux/exportfs.h | export_operations |
2. struct super_operations
2.1 定义
c
// include/linux/fs/super_types.h:83
struct super_operations {
struct inode *(*alloc_inode)(struct super_block *sb);
void (*destroy_inode)(struct inode *inode);
void (*free_inode)(struct inode *inode);
void (*dirty_inode)(struct inode *inode, int flags);
int (*write_inode)(struct inode *inode, struct writeback_control *wbc);
int (*drop_inode)(struct inode *inode);
void (*evict_inode)(struct inode *inode);
void (*put_super)(struct super_block *sb);
int (*sync_fs)(struct super_block *sb, int wait);
int (*freeze_super)(struct super_block *sb, ...);
int (*thaw_super)(struct super_block *sb, ...);
int (*statfs)(struct dentry *dentry, struct kstatfs *kstatfs);
void (*umount_begin)(struct super_block *sb);
int (*show_options)(struct seq_file *seq, struct dentry *dentry);
long (*nr_cached_objects)(struct super_block *sb, struct shrink_control *sc);
long (*free_cached_objects)(struct super_block *sb, struct shrink_control *sc);
};2.2 关键回调
| 回调 | 用途 |
|---|---|
alloc_inode | 为文件系统分配新 inode |
evict_inode | 从内存中清除 inode |
write_inode | 将 inode 写回磁盘 |
put_super | 卸载时释放 superblock |
sync_fs | 同步文件系统元数据 |
3. struct inode_operations
3.1 定义
c
// include/linux/fs.h:2001
struct inode_operations {
struct dentry *(*lookup)(struct inode *dir, struct dentry *dentry,
unsigned int flags);
const char *(*get_link)(struct dentry *, struct inode *,
struct delayed_call *);
int (*permission)(struct mnt_idmap *idmap, struct inode *inode, int mask);
struct posix_acl *(*get_acl)(struct mnt_idmap *idmap,
struct dentry *dentry, int type);
int (*create)(struct mnt_idmap *idmap, struct inode *dir,
struct dentry *dentry, umode_t mode, bool excl);
int (*link)(struct dentry *old_dentry, struct inode *dir,
struct dentry *dentry);
int (*unlink)(struct inode *dir, struct dentry *dentry);
int (*symlink)(struct mnt_idmap *idmap, struct inode *dir,
struct dentry *dentry, const char *oldname);
struct dentry *(*mkdir)(struct mnt_idmap *idmap, struct inode *dir,
struct dentry *dentry, umode_t mode);
int (*rmdir)(struct inode *dir, struct dentry *dentry);
int (*mknod)(struct mnt_idmap *idmap, struct inode *dir,
struct dentry *dentry, umode_t mode, dev_t dev);
int (*rename)(struct mnt_idmap *idmap, struct inode *old_dir,
struct dentry *old_dentry, struct inode *new_dir,
struct dentry *new_dentry, unsigned int flags);
int (*setattr)(struct mnt_idmap *idmap, struct dentry *dentry,
struct iattr *attr);
int (*getattr)(struct mnt_idmap *idmap, const struct path *path,
struct kstat *stat, u32 request_mask,
unsigned int flags);
ssize_t (*listxattr)(struct dentry *dentry, char *list, size_t size);
int (*atomic_open)(struct inode *dir, struct dentry *dentry,
struct file *file, unsigned open_flag, umode_t create_mode);
int (*tmpfile)(struct mnt_idmap *idmap, struct inode *dir,
struct file *file, umode_t mode);
};3.2 关键回调
| 回调 | 用途 |
|---|---|
lookup | 目录查找，返回匹配的 dentry |
create | 创建普通文件 |
mkdir | 创建目录 |
unlink | 删除文件 |
rename | 重命名/移动 |
permission | 权限检查 |
4. struct file_operations
4.1 定义
c
// include/linux/fs.h:1926
struct file_operations {
struct module *owner;
loff_t (*llseek)(struct file *filp, loff_t offset, int whence);
ssize_t (*read)(struct file *filp, char __user *buf,
size_t count, loff_t *pos);
ssize_t (*write)(struct file *filp, const char __user *buf,
size_t count, loff_t *pos);
ssize_t (*read_iter)(struct kiocb *iocb, struct iov_iter *to);
ssize_t (*write_iter)(struct kiocb *iocb, struct iov_iter *from);
int (*iterate_shared)(struct file *filp, struct dir_context *ctx);
__poll_t (*poll)(struct file *filp, struct poll_table_struct *pt);
long (*unlocked_ioctl)(struct file *filp, unsigned int cmd,
unsigned long arg);
long (*compat_ioctl)(struct file *filp, unsigned int cmd,
unsigned long arg);
int (*mmap)(struct file *filp, struct vm_area_struct *vma);
int (*open)(struct inode *inode, struct file *filp);
int (*flush)(struct file *filp, fl_owner_t id);
int (*release)(struct inode *inode, struct file *filp);
int (*fsync)(struct file *filp, loff_t start, loff_t end,
int datasync);
int (*fasync)(int fd, struct file *filp, int on);
ssize_t (*splice_read)(struct file *in, loff_t *ppos,
struct pipe_inode_info *pipe,
size_t len, unsigned int flags);
ssize_t (*splice_write)(struct pipe_inode_info *pipe,
struct file *out, loff_t *ppos,
size_t len, unsigned int flags);
int (*setlease)(struct file *filp, long arg, struct file_lease **lease,
void **priv);
long (*fallocate)(struct file *filp, int mode, loff_t offset,
loff_t len);
ssize_t (*copy_file_range)(struct file *file_in, loff_t pos_in,
struct file *file_out, loff_t pos_out,
size_t len, unsigned int flags);
};4.2 关键回调
| 回调 | 用途 |
|---|---|
llseek | 文件定位 |
read/read_iter | 读数据 |
write/write_iter | 写数据 |
open | 打开文件 |
release | 关闭文件 |
fsync | 同步到磁盘 |
5. struct address_space_operations
5.1 定义
c
// include/linux/fs.h:403
struct address_space_operations {
int (*read_folio)(struct file *filp, struct folio *folio);
int (*writepages)(struct address_space *mapping,
struct writeback_control *wbc);
bool (*dirty_folio)(struct address_space *mapping, struct folio *folio);
void (*readahead)(struct readahead_control *rac);
int (*write_begin)(struct kiocb *iocb, struct address_space *mapping,
loff_t pos, unsigned len,
struct folio **folio_ret, void **private_ret);
int (*write_end)(struct kiocb *iocb, struct address_space *mapping,
loff_t pos, unsigned copied, unsigned len,
struct folio *folio, void *private);
sector_t (*bmap)(struct address_space *mapping, sector_t block);
void (*invalidate_folio)(struct folio *folio, size_t offset,
size_t length);
bool (*release_folio)(struct folio *folio, gfp_t gfp_mask);
void (*free_folio)(struct folio *folio);
ssize_t (*direct_IO)(struct kiocb *iocb, struct iov_iter *iter);
int (*migrate_folio)(struct address_space *mapping,
struct folio *dst, struct folio *src,
enum migrate_mode mode);
int (*launder_folio)(struct folio *folio);
bool (*is_partially_uptodate)(struct folio *folio,
size_t from, size_t count);
int (*error_remove_folio)(struct address_space *mapping,
struct folio *folio);
};5.2 关键回调
| 回调 | 用途 |
|---|---|
read_folio | 读取文件页到缓存 |
write_begin/end | 写操作准备和完成 |
writepages | 批量写回脏页 |
bmap | 逻辑块到物理块映射 |
direct_IO | 直接 I/O (绕过缓存) |
6. struct dentry_operations
6.1 定义
c
// include/linux/dcache.h:151
struct dentry_operations {
int (*d_revalidate)(struct dentry *dentry, unsigned int flags);
int (*d_weak_revalidate)(struct dentry *dentry, unsigned int flags);
int (*d_hash)(const struct dentry *parent, struct qstr *name);
int (*d_compare)(const struct dentry *parent, unsigned int len,
const char *str, const struct qstr *name);
int (*d_delete)(const struct dentry *dentry);
void (*d_release)(struct dentry *dentry);
void (*d_prune)(struct dentry *dentry);
void (*d_iput)(struct dentry *dentry, struct inode *inode);
char *(*d_dname)(struct dentry *dentry, char *buffer, int buflen);
struct vfsmount *(*d_automount)(struct path *path);
int (*d_manage)(const struct path *path, bool rcu_walk);
struct dentry *(*d_real)(struct dentry *dentry,
enum d_real_type type);
};6.2 关键回调
| 回调 | 用途 |
|---|---|
d_revalidate | 重新验证 dentry 有效性 |
d_hash/d_compare | 文件名哈希和比较 |
d_delete | 引用归零时调用 |
d_iput | 解除 dentry 与 inode 关联 |
d_automount | 自动挂载触发 |
7. struct export_operations
7.1 定义
c
// include/linux/exportfs.h:281
struct export_operations {
int (*encode_fh)(struct inode *inode, __u32 *fh, int *max_len,
struct inode *parent);
struct dentry *(*fh_to_dentry)(struct super_block *sb, struct fid *fid,
int fh_len, int fh_type);
struct dentry *(*fh_to_parent)(struct super_block *sb, struct fid *fid,
int fh_len, int fh_type);
int (*get_name)(struct dentry *parent, char *name,
struct dentry *child);
struct dentry *(*get_parent)(struct dentry *dentry);
int (*commit_metadata)(struct inode *inode);
};8. 使用示例
8.1 ext4 文件系统操作设置
c
// fs/ext4/super.c
static const struct super_operations ext4_sops = {
.alloc_inode = ext4_alloc_inode,
.destroy_inode = ext4_destroy_inode,
.write_inode = ext4_write_inode,
.evict_inode = ext4_evict_inode,
.put_super = ext4_put_super,
.sync_fs = ext4_sync_fs,
.statfs = ext4_statfs,
.show_options = ext4_show_options,
};
// fs/ext4/namei.c
static const struct inode_operations ext4_dir_inode_operations = {
.create = ext4_create,
.lookup = ext4_lookup,
.link = ext4_link,
.unlink = ext4_unlink,
.symlink = ext4_symlink,
.mkdir = ext4_mkdir,
.rmdir = ext4_rmdir,
.rename = ext4_rename2,
.setattr = ext4_setattr,
.getattr = ext4_getattr,
.listxattr = ext4_listxattr,
.get_acl = ext4_get_acl,
.set_acl = ext4_set_acl,
};8.2 VFS 调用示例
c
// VFS 调用 inode_operations->lookup
// fs/namei.c
static int lookup_fast(struct nameidata *nd)
{
struct dentry *dentry;
dentry = d_lookup(nd->path.dentry, &nd->last);
if (dentry)
return 0;
// 调用文件系统 lookup
inode = nd->path.dentry->d_inode;
return inode->i_op->lookup(inode, dentry, nd->flags);
}