Linux Kernel系列 - VFS核心数据结构

Hanks.Wang - 专注于操作系统与移动安全研究，Linux-Kernel/SELinux/SEAndroid/TrustZone/Encription/MDM Mail - byhankswang@gmail.com

VFS核心数据结构

本文中参照的Linux Kernel版本为3.4.5
URL: https://www.kernel.org/pub/linux/kernel/v3.x/linux-3.4.5.tar.gz


阅读Linux文件系统源代码的重要性

Linux VFS通过抽象的手段对多种文件系统类型均有良好的支持，通过阅读VFS源码可以很好的学习使用C语言开发面向对象的大型项目；Linux系统中本身对物理设备抽象成文件的设计，通过阅读VFS源代码有助于深度理解设备管理和文件系统关系；Linux程序开发中大量的glibc API的实现都和VFS有一定的关系，通过阅读VFS源代码有助于理解和帮助用户态和内核态程序的开发；由于VFS在内核中的重要地位，通过阅读VFS源代码有助于理解内核中其他模块的实现。

VFS中比较重要的数据结构包括struct super_block, struct file_system_type, struct super_operations, struct dentry,struct inode, struct inode_operations, struct file, struct file_operations ;其中相关的基本数据结构请认真阅读linux源码文件include/linux/fs.h




图1 - linux文件系统体系结构

1.超级块结构体 super_block

struct super_block定义在include/linux/fs.h:1418

struct super_block {

struct list_heads_list;/* Keep this first */

dev_t
s_dev;/* search index; _not_ kdev_t */

unsigned char s_dirt;

unsigned char s_blocksize_bits;

unsigned long s_blocksize;

loff_ts_maxbytes;/* Max file size */

struct file_system_type* s_type;

const struct super_operations* s_op;

const struct dquot_operations* dq_op;

const struct quotactl_ops* s_qcop;

const struct export_operations * s_export_op;

unsigned longs_flags;

unsigned longs_magic;

struct dentry*s_root;

struct rw_semaphore s_umount;

struct mutex s_lock;

ints_count;

atomic_t s_active;

#ifdef CONFIG_SECURITY

void *s_security;

#endif

const struct xattr_handler ** s_xattr;

struct list_heads_inodes;/* all inodes */

struct hlist_bl_head s_anon;/* anonymous dentries for (nfs) exporting */

#ifdef CONFIG_SMP

struct list_head __percpu *s_files;

#else

struct list_heads_files;

#endif

struct list_heads_mounts; /* list of mounts; _not_ for fs use */

/* s_dentry_lru, s_nr_dentry_unused protected by dcache.c lru locks */

struct list_heads_dentry_lru;/* unused dentry lru */

ints_nr_dentry_unused;/* # of dentry on lru */

/* s_inode_lru_lock protects s_inode_lru and s_nr_inodes_unused */

spinlock_t s_inode_lru_lock ____cacheline_aligned_in_smp;

struct list_head s_inode_lru;/* unused inode lru */

ints_nr_inodes_unused;/* # of inodes on lru */

struct block_device* s_bdev;

struct backing_dev_info * s_bdi;

struct mtd_info* s_mtd;

struct hlist_node s_instances;

struct quota_info s_dquot;/* Diskquota specific options */

int
s_frozen;

wait_queue_head_ts_wait_unfrozen;

char s_id[32];/* Informational name */

u8 s_uuid[16];/* UUID */

void * s_fs_info;/* Filesystem private info */

unsigned int s_max_links;

fmode_t
s_mode;

/* Granularity of c/m/atime in ns.

Cannot be worse than a second */

u32
s_time_gran;

/*

* The next field is for VFS *only*. No filesystems have any business

* even looking at it. You had been warned.

*/

struct mutex s_vfs_rename_mutex;/* Kludge */

/*

* Filesystem subtype. If non-empty the filesystem type field

* in /proc/mounts will be "type.subtype"

*/

char *s_subtype;

/*

* Saved mount options for lazy filesystems using

* generic_show_options()

*/

char __rcu *s_options;

const struct dentry_operations *s_d_op; /* default d_op for dentries */

/*

* Saved pool identifier for cleancache (-1 means none)

*/

int cleancache_poolid;

struct shrinker s_shrink;/* per-sb shrinker handle */

/* Number of inodes with nlink == 0 but still referenced */

atomic_long_t s_remove_count;

/* Being remounted read-only */

int s_readonly_remount;

};

2. 文件系统类型 file_system_type

struct file_system_type定义在include/linux/fs.h:1835

struct file_system_type{

const char *name;

int fs_flags;

struct dentry *(*mount) (struct file_system_type *, int,

const char *, void *);

void (*kill_sb) (struct super_block *);

struct module *owner;

struct file_system_type * next;

struct hlist_head fs_supers;

struct lock_class_key s_lock_key;

struct lock_class_key s_umount_key;

struct lock_class_key s_vfs_rename_key;

struct lock_class_key i_lock_key;

struct lock_class_key i_mutex_key;

struct lock_class_key i_mutex_dir_key;

};

3. 文件系统超级操作 super_operations

super_operations定义在include/linux/fs.h1864行

struct super_operations {

struct inode *(*alloc_inode)(struct super_block *sb);

void (*destroy_inode)(struct inode *);

void (*dirty_inode) (struct inode *, int flags);

int (*write_inode) (struct inode *, struct writeback_control *wbc);

int (*drop_inode) (struct inode *);

void (*evict_inode) (struct inode *);

void (*put_super) (struct super_block *);

void (*write_super) (struct super_block *);

int (*sync_fs)(struct super_block *sb, int wait);

int (*freeze_fs) (struct super_block *);

int (*unfreeze_fs) (struct super_block *);

int (*statfs) (struct dentry *, struct kstatfs *);

int (*remount_fs) (struct super_block *, int *, char *);

void (*umount_begin) (struct super_block *);

int (*show_options)(struct seq_file *, struct dentry *);

int (*show_devname)(struct seq_file *, struct dentry *);

int (*show_path)(struct seq_file *, struct dentry *);

int (*show_stats)(struct seq_file *, struct dentry *);

#ifdef CONFIG_QUOTA

ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);

ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);

#endif

int (*bdev_try_to_free_page)(struct super_block*, struct page*, gfp_t);

int (*nr_cached_objects)(struct super_block *);

void (*free_cached_objects)(struct super_block *, int);

};

相关的参考文献：
https://www.ibm.com/developerworks/cn/aix/library/au-speakingunix14/