vfs概念

     Linux讀寫文件的一個demo如下,運行後系統死機了,原因是f->f_op->write爲NULL,導致空指針,將
f->f_op->write換成vfs_write代碼正常運行

static int stmvl53l0x_write_offset_calibration_file(void)
{
        struct file *f;
        char buf[8];
        mm_segment_t fs;

        f = filp_open("/productinfo/offset", O_WRONLY|O_CREAT, 0666);
        if (IS_ERR(f)) {
                vl53l0x_dbgmsg("unable to open /productinfo/offsetl\n");
                return PTR_ERR(f);
        }
        fs = get_fs();
        set_fs(KERNEL_DS);
        snprintf(buf, 8, "%d", offset_calib);
        vl53l0x_dbgmsg("write offset as:%s, buf[0]:%c\n", buf, buf[0]);
        f->f_op->write(f, buf, 8, &f->f_pos);
        set_fs(fs);
        filp_close(f, NULL);
        return 0;
}

ssize_t __vfs_write(struct file *file, const char __user *p, size_t count,
		    loff_t *pos)
{
	if (file->f_op->write)
		return file->f_op->write(file, p, count, pos);
	else if (file->f_op->write_iter)
		return new_sync_write(file, p, count, pos);
	else
		return -EINVAL;
}
ssize_t vfs_write(struct file *file, const char __user *buf, size_t count, loff_t *pos)
{
	ret = __vfs_write(file, buf, count, pos);
}

也就是說有些文件是write接口，有些是write_iter
對於ext4

const struct file_operations ext4_file_operations = {
	.llseek		= ext4_llseek,
	.read_iter	= generic_file_read_iter,
	.write_iter	= ext4_file_write_iter,
	.unlocked_ioctl = ext4_ioctl,
#ifdef CONFIG_COMPAT
	.compat_ioctl	= ext4_compat_ioctl,
#endif
	.mmap		= ext4_file_mmap,
	.open		= ext4_file_open,
	.release	= ext4_release_file,
	.fsync		= ext4_sync_file,
	.splice_read	= generic_file_splice_read,
	.splice_write	= iter_file_splice_write,
	.fallocate	= ext4_fallocate,
};

對於procfs

static const struct file_operations proc_reg_file_ops = {
	.llseek		= proc_reg_llseek,
	.read		= proc_reg_read,
	.write		= proc_reg_write,
	.poll		= proc_reg_poll,
	.unlocked_ioctl	= proc_reg_unlocked_ioctl,
#ifdef CONFIG_COMPAT
	.compat_ioctl	= proc_reg_compat_ioctl,
#endif
	.mmap		= proc_reg_mmap,
	.get_unmapped_area = proc_reg_get_unmapped_area,
	.open		= proc_reg_open,
	.release	= proc_reg_release,
};

也就是不同的文件系統，相應的file_ops有差異,所以有了VFS(Virtual File System)