Short Notes on File System


Data structures

  • file a file struct represents a physical file’s context, different processes, or even the same process can open one file multiple times, therefore has multiple file struct.
  • dentry multiple dentry can point to the same inode
  • inode one-to-one mapping with the real file on disk

Process and file

  • task_struct struct contains struct fs_struct *fs and struct files_struct *files
  • fs_struct defines the root directory of this process
  • files_struct maintains a list of file struct than opened by this process

task_struct fs

Figure credit to Linux内核源代码情景分析

Page cache

  • Page cache in kernel is organized per physical file, so it is contained in the inode, which is one-to-one mapping to physical file
  • inode contains i_mapping, which is address_space and contains the page cache for that file.
    struct address_space {
      struct inode		*host;		/* owner: inode, block_device */
      struct radix_tree_root	page_tree;	/* radix tree of all pages */
      spinlock_t		tree_lock;	/* and lock protecting it */
      unsigned int		i_mmap_writable;/* count VM_SHARED mappings */
      struct prio_tree_root	i_mmap;		/* tree of private and shared mappings */
      struct list_head	i_mmap_nonlinear;/*list VM_NONLINEAR mappings */
      struct mutex		i_mmap_mutex;	/* protect tree, count, list */
      /* Protected by tree_lock together with the radix tree */
      unsigned long		nrpages;	/* number of total pages */
      pgoff_t			writeback_index;/* writeback starts here */
      const struct address_space_operations *a_ops;	/* methods */
      unsigned long		flags;		/* error bits/gfp mask */
      struct backing_dev_info *backing_dev_info; /* device readahead, etc */
      spinlock_t		private_lock;	/* for use by the address_space */
      struct list_head	private_list;	/* ditto */
      struct address_space	*assoc_mapping;	/* ditto */
    } __attribute__((aligned(sizeof(long))));
  • page_tree contains the page cache, which is indexed by using radix tree. More details of page cache is in Understanding the Linux kernel, Chapter 15. page_cache_tree

Figure credit to Understanding the Linux Kernel

  • In order to be compatible with the underline block based device, the page cache all need to store the block, each page needs to point to a cyclic linked list with four buffer heads buffer_head

Figure credit to Understanding the Linux Kernel


  1. Understanding the Linux Kernel
  2. Linux内核源代码情景分析