Short Notes on File System

• Data structures
• Process and file
• Page cache
• References

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

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.

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

Figure credit to Understanding the Linux Kernel

References

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