inode具有一些标记文件状态的标志位。

/*
 * Inode state bits.  Protected by inode->i_lock
 *
 * Three bits determine the dirty state of the inode, I_DIRTY_SYNC,
 * I_DIRTY_DATASYNC and I_DIRTY_PAGES.
 *
 * Four bits define the lifetime of an inode.  Initially, inodes are I_NEW,
 * until that flag is cleared.  I_WILL_FREE, I_FREEING and I_CLEAR are set at
 * various stages of removing an inode.
 *
 * Two bits are used for locking and completion notification, I_NEW and I_SYNC.
 *
 * I_DIRTY_SYNC		Inode is dirty, but doesn't have to be written on
 *			fdatasync().  i_atime is the usual cause.
 * I_DIRTY_DATASYNC	Data-related inode changes pending. We keep track of
 *			these changes separately from I_DIRTY_SYNC so that we
 *			don't have to write inode on fdatasync() when only
 *			mtime has changed in it.
 * I_DIRTY_PAGES	Inode has dirty pages.  Inode itself may be clean.
 * I_NEW		Serves as both a mutex and completion notification.
 *			New inodes set I_NEW.  If two processes both create
 *			the same inode, one of them will release its inode and
 *			wait for I_NEW to be released before returning.
 *			Inodes in I_WILL_FREE, I_FREEING or I_CLEAR state can
 *			also cause waiting on I_NEW, without I_NEW actually
 *			being set.  find_inode() uses this to prevent returning
 *			nearly-dead inodes.
 * I_WILL_FREE		Must be set when calling write_inode_now() if i_count
 *			is zero.  I_FREEING must be set when I_WILL_FREE is
 *			cleared.
 * I_FREEING		Set when inode is about to be freed but still has dirty
 *			pages or buffers attached or the inode itself is still
 *			dirty.
 * I_CLEAR		Added by clear_inode().  In this state the inode is
 *			clean and can be destroyed.  Inode keeps I_FREEING.
 *
 *			Inodes that are I_WILL_FREE, I_FREEING or I_CLEAR are
 *			prohibited for many purposes.  iget() must wait for
 *			the inode to be completely released, then create it
 *			anew.  Other functions will just ignore such inodes,
 *			if appropriate.  I_NEW is used for waiting.
 *
 * I_SYNC		Writeback of inode is running. The bit is set during
 *			data writeback, and cleared with a wakeup on the bit
 *			address once it is done. The bit is also used to pin
 *			the inode in memory for flusher thread.
 *
 * I_REFERENCED		Marks the inode as recently references on the LRU list.
 *
 * I_DIO_WAKEUP		Never set.  Only used as a key for wait_on_bit().
 *
 * Q: What is the difference between I_WILL_FREE and I_FREEING?
 */

用于fsync

I_DIRTY_SYNC、I_DIRTY_DATASYNC和I_DIRTY_PAGES用于标记这个文件的inode是否脏。

• I_DIRTY_SYNC表示inode的访问时间等不影响数据一致性的属性脏了。仅有这个标记的inode不需要在fdatasync的时候刷回磁盘。
• I_DIRTY_DATASYNC表示inode的修改时间等和数据一致性有关的属性脏了。有这个标记的inode需要在fdatasync的时候刷盘。
• I_DIRTY_PAGES表示inode对应的文件数据发生了修改，page cache中有脏页。但是inode本身可能是干净的。

I_DIRTY是I_DIRTY_SYNC | I_DIRTY_DATASYNC | I_DIRTY_PAGES。只要文件发生了任何变化，就具有I_DIRTY标记。