file オブジェクトは Linux や POSIX 系のファイル関数で扱うファイルを抽象化したものです。

　プロセスが開くファイルの抽象化であり、ファイル記述子をまとめたテーブルである fdtable のエントリーになります。

// /usr/src/linux-headers-4.18.16-041816-generic/include/linux/fdtable.h
 26 struct fdtable {
 27   unsigned int max_fds;
 28   struct file __rcu **fd;      /* current fd array */
 29   unsigned long *close_on_exec;
 30   unsigned long *open_fds;
 31   unsigned long *full_fds_bits;
 32   struct rcu_head rcu;
 33 };
//割愛
 45 /*
 46  * Open file table structure
 47  */
 48 struct files_struct {
 49   /*
 50    * read mostly part
 51    */
 52   atomic_t count;
 53   bool resize_in_progress;
 54   wait_queue_head_t resize_wait;
 55
 56   struct fdtable __rcu *fdt;
 57   struct fdtable fdtab;
 58   /*
 59    * written part on a separate cache line in SMP
 60    */
 61   spinlock_t file_lock ____cacheline_aligned_in_smp;
 62   unsigned int next_fd;
 63   unsigned long close_on_exec_init[1];
 64   unsigned long open_fds_init[1];
 65   unsigned long full_fds_bits_init[1];
 66   struct file __rcu * fd_array[NR_OPEN_DEFAULT];
 67 };
 68

　file オブジェクトの宣言は以下のようになります。

// /usr/src/linux-headers-4.18.16-041816-generic/include/linux/fs.h
 859 struct file {
 860   union {
 861     struct llist_node fu_llist;
 862     struct rcu_head   fu_rcuhead;
 863   } f_u;
 864   struct path   f_path;
 865   struct inode    *f_inode; /* cached value */
 866   const struct file_operations  *f_op;
 867
 868   /*
 869    * Protects f_ep_links, f_flags.
 870    * Must not be taken from IRQ context.
 871    */
 872   spinlock_t    f_lock;
 873   enum rw_hint    f_write_hint;
 874   atomic_long_t   f_count;
 875   unsigned int    f_flags;
 876   fmode_t     f_mode;
 877   struct mutex    f_pos_lock;
 878   loff_t      f_pos;
 879   struct fown_struct  f_owner;
 880   const struct cred *f_cred;
 881   struct file_ra_state  f_ra;
 882
 883   u64     f_version;
 884 #ifdef CONFIG_SECURITY
 885   void      *f_security;
 886 #endif
 887   /* needed for tty driver, and maybe others */
 888   void      *private_data;
 889
 890 #ifdef CONFIG_EPOLL
 891   /* Used by fs/eventpoll.c to link all the hooks to this file */
 892   struct list_head  f_ep_links;
 893   struct list_head  f_tfile_llink;
 894 #endif /* #ifdef CONFIG_EPOLL */
 895   struct address_space  *f_mapping;
 896   errseq_t    f_wb_err;
 897 } __randomize_layout
 898   __attribute__((aligned(4)));  /* lest something weird decides that 2 is OK */

　file オブジェクトの操作には file_operations オブジェクトに宣言された関数ポインターを使います。

1713 struct file_operations {
1714   struct module *owner;
1715   loff_t (*llseek) (struct file *, loff_t, int);
1716   ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
1717   ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
1718   ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
1719   ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
1720   int (*iterate) (struct file *, struct dir_context *);
1721   int (*iterate_shared) (struct file *, struct dir_context *);
1722   __poll_t (*poll) (struct file *, struct poll_table_struct *);
1723   long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
1724   long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
1725   int (*mmap) (struct file *, struct vm_area_struct *);
1726   unsigned long mmap_supported_flags;
1727   int (*open) (struct inode *, struct file *);
1728   int (*flush) (struct file *, fl_owner_t id);
1729   int (*release) (struct inode *, struct file *);
1730   int (*fsync) (struct file *, loff_t, loff_t, int datasync);
1731   int (*fasync) (int, struct file *, int);
1732   int (*lock) (struct file *, int, struct file_lock *);
1733   ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
1734   unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned
1735   int (*check_flags)(int);
1736   int (*flock) (struct file *, int, struct file_lock *);
1737   ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsig
1738   ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsign
1739   int (*setlease)(struct file *, long, struct file_lock **, void **);
1740   long (*fallocate)(struct file *file, int mode, loff_t offset,
1741         loff_t len);
1742   void (*show_fdinfo)(struct seq_file *m, struct file *f);
1743 #ifndef CONFIG_MMU
1744   unsigned (*mmap_capabilities)(struct file *);
1745 #endif
1746   ssize_t (*copy_file_range)(struct file *, loff_t, struct file *,
1747       loff_t, size_t, unsigned int);
1748   int (*clone_file_range)(struct file *, loff_t, struct file *, loff_t,
1749       u64);
1750   ssize_t (*dedupe_file_range)(struct file *, u64, u64, struct file *,
1751       u64);
1752 } __randomize_layout;

　この中には mmap 関数も含まれています。

1713 struct file_operations {
1725   int (*mmap) (struct file *, struct vm_area_struct *);
1752 } __randomize_layout;

　この mmap 関数はカーネル側（デバイスドライバー） の処理を担当しているので mmap システムコールの宣言とは随分違いますね。

#include <sys/mman.h>
void *mmap(void *addr, size_t length, int prot, int flags,
                  int fd, off_t offset);

　まあ file_operations の中の mmap は（ブロック）デバイス空間とユーザー空間のマッピングとなりますんで、ファイル記述子が呼び出しに必要となっています。

　さらに引数の vm_area_struct へのポインターはどの仮想アドレスにマップできるかを指定させてくれます。