文件打開了，我們就來寫一寫。

write

系統(tǒng)調(diào)用write定義在kernel/fs/read_write.c，通過vfs_write調(diào)用file->f_op->write來寫入文件。

SYSCALL_DEFINE3(write, unsigned int, fd, const char __user *, buf,
        size_t, count)
{
    struct fd f = fdget_pos(fd);
    ssize_t ret = -EBADF;

    if (f.file) {
        loff_t pos = file_pos_read(f.file);
        ret = vfs_write(f.file, buf, count, &pos);
        if (ret >= 0)
            file_pos_write(f.file, pos);
        fdput_pos(f);
    }

    return ret;
}

FAT的文件操作如下，定義在kernel/fs/fat/file.c，可以看到.write=new_sync_write，即file->f_op->write=new_sync_write。
new_sync_write又調(diào)用了filp->f_op->write_iter，即generic_file_write_iter。

const struct file_operations fat_file_operations = {
    .llseek     = generic_file_llseek,
    .read       = new_sync_read,
    .write      = new_sync_write,
    .read_iter  = generic_file_read_iter,
    .write_iter = generic_file_write_iter,
    .mmap       = generic_file_mmap,
    .release    = fat_file_release,
    .unlocked_ioctl = fat_generic_ioctl,
#ifdef CONFIG_COMPAT
    .compat_ioctl   = fat_generic_compat_ioctl,
#endif
    .fsync      = fat_file_fsync,
    .splice_read    = generic_file_splice_read,
};

generic_file_write_iter定義在kernel/mm/filemap.c，進一步是__generic_file_write_iter==>generic_perform_write。

ssize_t generic_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
{
    struct file *file = iocb->ki_filp;
    struct inode *inode = file->f_mapping->host;
    ssize_t ret;

    mutex_lock(&inode->i_mutex);
    ret = __generic_file_write_iter(iocb, from);
    mutex_unlock(&inode->i_mutex);

    if (ret > 0) {
        ssize_t err;

        err = generic_write_sync(file, iocb->ki_pos - ret, ret);
        if (err < 0)
            ret = err;
    }
    return ret;
}

簡化如下，iov_iter_copy_from_user_atomic就是一個類似于memcpy的函數(shù)，它把用戶buffer的內(nèi)容復(fù)制到內(nèi)存頁中。

ssize_t generic_perform_write(struct file *file,
                struct iov_iter *i, loff_t pos)
{
    //...
    copied = iov_iter_copy_from_user_atomic(page, i, offset, bytes);
    //...
}

其實到這里，一般的write流程就結(jié)束了，所謂內(nèi)存頁也只是某塊內(nèi)存，也就是write結(jié)束時，內(nèi)容并沒有寫到磁盤中。如果這時候突然斷電，磁盤里是沒有更新的。

內(nèi)存里的臟buffer什么時候刷到磁盤上的呢？