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1. struct address_space {
2.     struct inode        *host;        /* owner: inode, block_device */
3.     struct radix_tree_root    page_tree;    /* radix tree of all pages */
4.     spinlock_t        tree_lock;    /* and lock protecting it */
5.     atomic_t        i_mmap_writable;/* count VM_SHARED mappings */
6.     struct rb_root_cached    i_mmap;        /* tree of private and shared mappings */
7.     struct rw_semaphore    i_mmap_rwsem;    /* protect tree, count, list */
8.     /* Protected by tree_lock together with the radix tree */
9.     unsigned long        nrpages;    /* number of total pages */
10.     /* number of shadow or DAX exceptional entries */
11.     unsigned long        nrexceptional;
12.     pgoff_t            writeback_index;/* writeback starts here */
13.     const struct address_space_operations *a_ops;    /* methods */
14.     unsigned long        flags;        /* error bits */
15.     spinlock_t        private_lock;    /* for use by the address_space */
16.     gfp_t            gfp_mask;    /* implicit gfp mask for allocations */
17.     struct list_head    private_list;    /* for use by the address_space */
18.     void            *private_data;    /* ditto */
19.     errseq_t        wb_err;
20. } __attribute__((aligned(sizeof(long)))) __randomize_layout;
21.  
22. struct address_space_operations {
23.     int (*writepage)(struct page *page, struct writeback_control *wbc);
24.     int (*readpage)(struct file *, struct page *);
25.  
26.     /* Write back some dirty pages from this mapping. */
27.     int (*writepages)(struct address_space *, struct writeback_control *);
28.  
29.     /* Set a page dirty. Return true if this dirtied it */
30.     int (*set_page_dirty)(struct page *page);
31.  
32.     int (*readpages)(struct file *filp, struct address_space *mapping,
33.             struct list_head *pages, unsigned nr_pages);
34.  
35.     int (*write_begin)(struct file *, struct address_space *mapping,
36.                 loff_t pos, unsigned len, unsigned flags,
37.                 struct page **pagep, void **fsdata);
38.     int (*write_end)(struct file *, struct address_space *mapping,
39.                 loff_t pos, unsigned len, unsigned copied,
40.                 struct page *page, void *fsdata);
41.  
42.     /* Unfortunately this kludge is needed for FIBMAP. Don't use it */
43.     sector_t (*bmap)(struct address_space *, sector_t);
44.     void (*invalidatepage) (struct page *, unsigned int, unsigned int);
45.     int (*releasepage) (struct page *, gfp_t);
46.     void (*freepage)(struct page *);
47.     ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter);
48.     /*
49.      * migrate the contents of a page to the specified target. If
50.      * migrate_mode is MIGRATE_ASYNC, it must not block.
51.      */
52.     int (*migratepage) (struct address_space *,
53.             struct page *, struct page *, enum migrate_mode);
54.     bool (*isolate_page)(struct page *, isolate_mode_t);
55.     void (*putback_page)(struct page *);
56.     int (*launder_page) (struct page *);
57.     int (*is_partially_uptodate) (struct page *, unsigned long,
58.                     unsigned long);
59.     void (*is_dirty_writeback) (struct page *, bool *, bool *);
60.     int (*error_remove_page)(struct address_space *, struct page *);
61.  
62.     /* swapfile support */
63.     int (*swap_activate)(struct swap_info_struct *sis, struct file *file,
64.                 sector_t *span);
65.     void (*swap_deactivate)(struct file *file);
66. };

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1. address_space_operations当中的readpage函数)
2. const struct vm_operations_struct generic_file_vm_ops = {
3.     .fault        = filemap_fault,
4.     .map_pages    = filemap_map_pages,
5.     .page_mkwrite    = filemap_page_mkwrite,
6. };
7. int generic_file_mmap(struct file * file, struct vm_area_struct * vma)
8. {
9.     struct address_space *mapping = file->f_mapping;
10.  
11.     if (!mapping->a_ops->readpage)
12.         return -ENOEXEC;
13.     file_accessed(file);
14.     vma->vm_ops = &generic_file_vm_ops;
15.     return 0;
16. }
17.