文件系统

文件系统分析（从路径到目标节点）

struct fs_struct{
atomic_t count;
rwlock_t lock;
int mask;
struct dentry *root,*pwd,*altroot;
struct vfsmount *rootmnt,*pwdmnt,*altrootmnt;
};
fs_struct结构中的信息都是与文件系统和进程相关 与具体的已打开的文件没有关系

打开的文件表数据结构
struct files_struct{
atomic_t count;
spinlock_t file_lock;
int max_fds;
int max_fdset;
int next fd;
struct file **fd;
fd_set *close_on_exec;
fd_set open_fds;
fd_set close_on_exec_init;
fd_set open_fds_init;
struct file *fd_array[NR_OPEN_DEFAULT];
};

struct ext2_inode 把inode结构的部分信息保存在磁盘的索引节点上。 在内存中对应为inode结构

目录项 (索引节点的数据项）  在内存中对应为 dentry 结构
struct ext2_dir_entry_2{
__u32 inode;
__u16 rec_len;
__u8  name_len;
__u8 file_type;
char name[EXT2_NAME_LEN];
};
每个磁盘分区都有一个超级块

打开文件的过程：
1.系统调用 sys_open
asmlinkage long sys_open(const char __user * filename, int flags, int mode)
{;
...struct file *f = filp_open(tmp, flags, mode);  //tmp 文件名
}
2.struct file *filp_open(const char * filename, int flags, int mode)

 error = open_namei(filename, namei_flags, mode, &nd);
 if (!error)
  return dentry_open(nd.dentry, nd.mnt, flags);

3.int open_namei(const char * pathname, int flag, int mode, struct nameidata *nd)
{ error = path_lookup(pathname, lookup_flags(flag)|LOOKUP_OPEN, nd);
}
4.int fastcall path_lookup(const char *name, unsigned int flags, struct nameidata *nd)
{
retval = link_path_walk(name, nd);
}
5,result = __link_path_walk(name, nd);
{
;err = do_lookup(nd, &this, &next); 进行真正的搜索
}
dentry结构是文件系统的核心数据结构，也是文件访问和为文件访问而做的文件路径搜索的枢纽。
每个dentry 结构都通过队列头d_hash 链入杂凑表 dentry_hashtable中的某个队列里
共享计数为0的dentry 通过d_lru 链入LRU list
dentry的数据结构
struct dentry {
 atomic_t d_count;
 unsigned int d_flags;  /* protected by d_lock */
 spinlock_t d_lock;  /* per dentry lock */
 struct inode *d_inode;  /* Where the name belongs to - NULL is
      * negative */
 /*
  * The next three fields are touched by __d_lookup.  Place them here
  * so they all fit in a 16-byte range, with 16-byte alignment.
  */
 struct dentry *d_parent; /* parent directory */
 struct qstr d_name;

 struct list_head d_lru;  /* LRU list */
 struct list_head d_child; /* child of parent list */
 struct list_head d_subdirs; /* our children */
 struct list_head d_alias; /* inode alias list */
 unsigned long d_time;  /* used by d_revalidate */
 struct dentry_operations *d_op;
 struct super_block *d_sb; /* The root of the dentry tree */
 void *d_fsdata;   /* fs-specific data */
  struct rcu_head d_rcu;
 struct dcookie_struct *d_cookie; /* cookie, if any */
 struct hlist_node d_hash; /* lookup hash list */ 
 int d_mounted;
 unsigned char d_iname[DNAME_INLINE_LEN_MIN]; /* small names */
};

static int do_lookup(struct nameidata *nd, struct qstr *name,
       struct path *path)
{
 struct vfsmount *mnt = nd->mnt;
 struct dentry *dentry = __d_lookup(nd->dentry, name);

 if (!dentry)
  goto need_lookup;
 if (dentry->d_op && dentry->d_op->d_revalidate)
  goto need_revalidate;
done:
 path->mnt = mnt;
 path->dentry = dentry;
 __follow_mount(path);
 return 0;

need_lookup:
 dentry = real_lookup(nd->dentry, name, nd);
 if (IS_ERR(dentry))
  goto fail;
 goto done;

need_revalidate:
 if (dentry->d_op->d_revalidate(dentry, nd))
  goto done;
 if (d_invalidate(dentry))
  goto done;
 dput(dentry);
 goto need_lookup;

fail:
 return PTR_ERR(dentry);
}