Linux当中的文件映射

文件的内存映射可以认为是两个不同的地址空间之间的映射，以简化程序员的工作。一个地址空间是用户进程的虚拟地址空间，另一个是文件系统所在的地址空间。


在内核创建一个映射时，必须建立两个地址空间之间的关联，以支持二者以读写请求的形式通信。vm_operations_struct结构即用于完成此项工作，它提供了一个操作，读取已经映射到虚拟地址空间，但是内容尚未进入物理内存的页。
但是该结构不了解映射类型，或性质的相关信息，只能算是一个通用接口，具体实现还需要内核描述打开文件的file结构体当中的成员变量address_space结构体来详细说明数据源。

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 */
    atomic_t        i_mmap_writable;/* count VM_SHARED mappings */
    struct rb_root_cached    i_mmap;        /* tree of private and shared mappings */
    struct rw_semaphore    i_mmap_rwsem;    /* protect tree, count, list */
    /* Protected by tree_lock together with the radix tree */
    unsigned long        nrpages;    /* number of total pages */
    /* number of shadow or DAX exceptional entries */
    unsigned long        nrexceptional;
    pgoff_t            writeback_index;/* writeback starts here */
    const struct address_space_operations *a_ops;    /* methods */
    unsigned long        flags;        /* error bits */
    spinlock_t        private_lock;    /* for use by the address_space */
    gfp_t            gfp_mask;    /* implicit gfp mask for allocations */
    struct list_head    private_list;    /* for use by the address_space */
    void            *private_data;    /* ditto */
    errseq_t        wb_err;
} __attribute__((aligned(sizeof(long)))) __randomize_layout;

struct address_space_operations {
    int (*writepage)(struct page *page, struct writeback_control *wbc);
    int (*readpage)(struct file *, struct page *);

    /* Write back some dirty pages from this mapping. */
    int (*writepages)(struct address_space *, struct writeback_control *);

    /* Set a page dirty. Return true if this dirtied it */
    int (*set_page_dirty)(struct page *page);

    int (*readpages)(struct file *filp, struct address_space *mapping,
            struct list_head *pages, unsigned nr_pages);

    int (*write_begin)(struct file *, struct address_space *mapping,
                loff_t pos, unsigned len, unsigned flags,
                struct page **pagep, void **fsdata);
    int (*write_end)(struct file *, struct address_space *mapping,
                loff_t pos, unsigned len, unsigned copied,
                struct page *page, void *fsdata);

    /* Unfortunately this kludge is needed for FIBMAP. Don't use it */
    sector_t (*bmap)(struct address_space *, sector_t);
    void (*invalidatepage) (struct page *, unsigned int, unsigned int);
    int (*releasepage) (struct page *, gfp_t);
    void (*freepage)(struct page *);
    ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter);
    /*
     * migrate the contents of a page to the specified target. If
     * migrate_mode is MIGRATE_ASYNC, it must not block.
     */
    int (*migratepage) (struct address_space *,
            struct page *, struct page *, enum migrate_mode);
    bool (*isolate_page)(struct page *, isolate_mode_t);
    void (*putback_page)(struct page *);
    int (*launder_page) (struct page *);
    int (*is_partially_uptodate) (struct page *, unsigned long,
                    unsigned long);
    void (*is_dirty_writeback) (struct page *, bool *, bool *);
    int (*error_remove_page)(struct address_space *, struct page *);

    /* swapfile support */
    int (*swap_activate)(struct swap_info_struct *sis, struct file *file,
                sector_t *span);
    void (*swap_deactivate)(struct file *file);
};

这样每个address_space都有一组相关操作，保存在自己的a_ops指针当中
当建立内存映射的时候，内核分配完新的vm_area_struct实例之后，就会用特定于文件的函数file->f_op->mmap创建映射，vm_area_struct当中的vm_operations_struct和address_space之间就会建立联系(vm_operations_struct 的指针最后指向的是generic_file_vm_ops,里面的fault函数指向filemap_fault，这个函数就会调用上

address_space_operations当中的readpage函数)
const struct vm_operations_struct generic_file_vm_ops = {
    .fault        = filemap_fault,
    .map_pages    = filemap_map_pages,
    .page_mkwrite    = filemap_page_mkwrite,
};
int generic_file_mmap(struct file * file, struct vm_area_struct * vma)
{
    struct address_space *mapping = file->f_mapping;

    if (!mapping->a_ops->readpage)
        return -ENOEXEC;
    file_accessed(file);
    vma->vm_ops = &generic_file_vm_ops;
    return 0;
}