VFS四大struct：file、dentry、inode、super_block
dentry
dentry有什么作用？dentry保存了目录/文件到inode的映射，在open一个文件时，如果路径上的某个目录或者最后的文件没有对应的dentry，则会为其建立dentry并插入dentry hashtable，dentry里的d_inode成员保存了该目录文件/文件的indoe指针。这样后面如果在lookup这个目录/文件时就可以根据目录/文件的hash值/name等找到对应的dentry，找到了对应的dentry后，则就知道了这个目录文件/文件的inode struct，就不用再进入文件系统层面去lookup这个目录/文件了，如果是进入到文件系统层面去lookup了，会去读目录文件lookup，而这个涉及到读IO，比较耗时。
dentry其实是包含路径含义的，比如说某个目录/文件的dentry，这里说的目录/文件是需要有其完整的路径的，比如/data/test/foo目录，/data/test/foo/bar.txt文件，因为dentry的hash值是根据其父目录的dentry和目录名/文件名两者计算出来的。
- d_name: qstr struct，里面存储了此dentry的hash值，此hash值用于在dentry hashtable上查找匹配；里面的name成员指向d_iname成员数组
- d_parent：此dentry的parent dentry
- d_iname: 保存的目录或者文件的名字，这个名字不包含路径，这个数组的大小是32（CONFIG_64BIT defined case）
struct dentry { /* RCU lookup touched fields */ unsigned int d_flags; /* protected by d_lock */ seqcount_t d_seq; /* per dentry seqlock */ struct hlist_bl_node d_hash; /* lookup hash list */ struct dentry *d_parent; /* parent directory */ struct qstr d_name; struct inode *d_inode; /* Where the name belongs to - NULL is * negative */ unsigned char d_iname[DNAME_INLINE_LEN]; /* small names */ /* Ref lookup also touches following */ struct lockref d_lockref; /* per-dentry lock and refcount */ const struct dentry_operations *d_op; struct super_block *d_sb; /* The root of the dentry tree */ unsigned long d_time; /* used by d_revalidate */ void *d_fsdata; /* fs-specific data */ union { struct list_head d_lru; /* LRU list */ wait_queue_head_t *d_wait; /* in-lookup ones only */ }; struct list_head d_child; /* child of parent list */ struct list_head d_subdirs; /* our children */ /* * d_alias and d_rcu can share memory */ union { struct hlist_node d_alias; /* inode alias list */ struct hlist_bl_node d_in_lookup_hash; /* only for in-lookup ones */ struct rcu_head d_rcu; } d_u; } __randomize_layout;
file struct
表示一个file
- f_path: 一个path结构体，这个path结构体里保存了这个文件的dentry，根据dentry又可以得到inode struct。path结构体里还有file所在的文件系统对应的vfs_mount struct；
- f_op：很重要的成员，read/write等系统调用就是根据这个file_operations来调用具体文件系统的file_operations函数；
struct file { union { struct llist_node fu_llist; struct rcu_head fu_rcuhead; } f_u; struct path f_path; struct inode *f_inode; /* cached value */ const struct file_operations *f_op; /* * Protects f_ep_links, f_flags. * Must not be taken from IRQ context. */ spinlock_t f_lock; enum rw_hint f_write_hint; atomic_long_t f_count; unsigned int f_flags; fmode_t f_mode; struct mutex f_pos_lock; loff_t f_pos; struct fown_struct f_owner; const struct cred *f_cred; struct file_ra_state f_ra; u64 f_version; #ifdef CONFIG_SECURITY void *f_security; #endif /* needed for tty driver, and maybe others */ void *private_data; #ifdef CONFIG_EPOLL /* Used by fs/eventpoll.c to link all the hooks to this file */ struct list_head f_ep_links; struct list_head f_tfile_llink; #endif /* #ifdef CONFIG_EPOLL */ struct address_space *f_mapping; errseq_t f_wb_err; } __randomize_layout __attribute__((aligned(4))); /* lest something weird decides that 2 is OK */
struct inode
文件目录都会有inode（目录也是一种文件，目录文件里的内容是其子目录/文件名及其inode number）
- i_fop，这个成员将会被赋值给file struct f_op成员，成为file struct里的file_operations函数集；
- i_ino，file inode number
struct inode { umode_t i_mode; unsigned short i_opflags; kuid_t i_uid; kgid_t i_gid; unsigned int i_flags; #ifdef CONFIG_FS_POSIX_ACL struct posix_acl *i_acl; struct posix_acl *i_default_acl; #endif const struct inode_operations *i_op; struct super_block *i_sb; struct address_space *i_mapping; #ifdef CONFIG_SECURITY void *i_security; #endif /* Stat data, not accessed from path walking */ unsigned long i_ino; /* * Filesystems may only read i_nlink directly. They shall use the * following functions for modification: * * (set|clear|inc|drop)_nlink * inode_(inc|dec)_link_count */ union { const unsigned int i_nlink; unsigned int __i_nlink; }; dev_t i_rdev; loff_t i_size; struct timespec64 i_atime; struct timespec64 i_mtime; struct timespec64 i_ctime; spinlock_t i_lock; /* i_blocks, i_bytes, maybe i_size */ unsigned short i_bytes; u8 i_blkbits; u8 i_write_hint; blkcnt_t i_blocks; #ifdef __NEED_I_SIZE_ORDERED seqcount_t i_size_seqcount; #endif /* Misc */ unsigned long i_state; struct rw_semaphore i_rwsem; unsigned long dirtied_when; /* jiffies of first dirtying */ unsigned long dirtied_time_when; struct hlist_node i_hash; struct list_head i_io_list; /* backing dev IO list */ #ifdef CONFIG_CGROUP_WRITEBACK struct bdi_writeback *i_wb; /* the associated cgroup wb */ /* foreign inode detection, see wbc_detach_inode() */ int i_wb_frn_winner; u16 i_wb_frn_avg_time; u16 i_wb_frn_history; #endif struct list_head i_lru; /* inode LRU list */ struct list_head i_sb_list; struct list_head i_wb_list; /* backing dev writeback list */ union { struct hlist_head i_dentry; struct rcu_head i_rcu; }; atomic64_t i_version; atomic64_t i_sequence; /* see futex */ atomic_t i_count; atomic_t i_dio_count; atomic_t i_writecount; #ifdef CONFIG_IMA atomic_t i_readcount; /* struct files open RO */ #endif const struct file_operations *i_fop; /* former ->i_op->default_file_ops */ struct file_lock_context *i_flctx; struct address_space i_data; struct list_head i_devices; union { struct pipe_inode_info *i_pipe; struct block_device *i_bdev; struct cdev *i_cdev; char *i_link; unsigned i_dir_seq; }; __u32 i_generation; #ifdef CONFIG_FSNOTIFY __u32 i_fsnotify_mask; /* all events this inode cares about */ struct fsnotify_mark_connector __rcu *i_fsnotify_marks; #endif #if IS_ENABLED(CONFIG_FS_ENCRYPTION) struct fscrypt_info *i_crypt_info; #endif void *i_private; /* fs or device private pointer */ } __randomize_layout;