虛擬檔案系統這一層提供的大多數是形如vfs_xxx的函式，他們最終呼叫的還是各個具體的檔案系統註冊給虛擬檔案系統的struct inode_operations和struct file_operations等結構體中的一些操作函式。以fat檔案系統為例，他註冊的struct inode_operations和struct file_operations如下：

//fs/fat/namei_vfat.c
static const struct inode_operations vfat_dir_inode_operations = {
        .create         = vfat_create,
        .lookup
 
         = vfat_lookup,
        .unlink         = vfat_unlink,
        .mkdir          = vfat_mkdir,
        .rmdir          = vfat_rmdir,
        .rename         = vfat_rename,
        .setattr        = fat_setattr,
        .getattr        = fat_getattr,
};

//fs/fat/file.c
const struct file_operations fat_file_operations = {
        .llseek
 
         = generic_file_llseek,
        .read           = do_sync_read,
        .write          = do_sync_write,
        .aio_read       = generic_file_aio_read,
        .aio_write      = generic_file_aio_write,
        .mmap           = generic_file_mmap,
        .release        = fat_file_release,
        .unlocked
 
_ioctl = fat_generic_ioctl,
        .fsync          = fat_file_fsync,
        .splice_read    = generic_file_splice_read,
};

open系統呼叫

//fs/open.c
SYSCALL_DEFINE3(open, const char __user *, filename, ...)
    do_sys_open
        do_filp_open //fs/namei.c
            path_openat
                do_last
                    lookup_open
                        vfs_create
                        finish_open
                            do_dentry_open
                                f->f_op = fops_get(inode->i_fop);

在open的時候會將inode中結構體struct file_operations i_fop儲存的函式指標賦值給file中的結構體struct file_operations f_op，以後read，write等函式呼叫的都是這裡的struct file_operations結構體中的回撥函式。

write系統呼叫

//fs/read_write.c
SYSCALL_DEFINE3(write, ...
    vfs_write
        file->f_op->write (fat檔案系統為do_sync_write)
        do_sync_write
            filp->f_op->aio_write (fat檔案系統為generic_file_aio_write)
            generic_file_aio_write //mm/filemap.c
                __generic_file_aio_write
                    generic_file_buffered_write
                        generic_perform_write
                            a_ops->write_begin
                            iov_iter_copy_from_user_atomic
                            a_ops->write_end

首先，系統呼叫write會呼叫虛擬檔案系統中的函式vfs_write，vfs_write最終會呼叫fat檔案系統註冊的struct file_operations函式中的f_op->aio_write函式，generic_file_aio_write是在mm/filemap.c檔案中實現的函式，這個函式會將資料寫到page cache中，等待回寫執行緒來將page cache寫到磁碟中。

mkdir系統呼叫

//fs/namei.c
SYSCALL_DEFINE2(mkdir, ...
    sys_mkdirat
SYSCALL_DEFINE3(mkdirat, ...
    vfs_mkdir(struct inode *dir, struct dentry *dentry, ...)
        dir->i_op->mkdir

mkdir系統呼叫與write系統呼叫不一樣，他會呼叫inode中的struct inode_operations結構體中的函式，而write系統呼叫呼叫的是file中的struct file_operations結構體中的函式。