[Devel] [PATCH VZ9] dm-qcow2: allow specifying depth to merge intermediate images

[Pavel Tikhomirov]

> @@ -152,25 +151,35 @@ static int qcow2_break_l1cow(struct qcow2_target *tgt)
>   static void set_backward_merge_in_process(struct qcow2_target *tgt,
>   				     struct qcow2 *qcow2, bool set)
>   {
> +	struct qcow2 *top = tgt->top;
>   	LIST_HEAD(list);
>   
> +	/*
> +	 * There are no writes if it is not the top qcow2 image.
> +	 * so we do not need to stop and flush requests when setting
> +	 */
> +	if (qcow2 != top && set) {
> +		qcow2->backward_merge_in_process = set;
> +		return;
> +	}
> +

Below hunks in this function feel excess to me. Why replace s/qcow2/top/ 
where qcow2 == top was just explicitly checked?

In that light, I would've removed top variable all together and used 
"qcow2 != tgt->top" directly, to simplify the code.

>   	/*
>   	 * To avoid race between allocations and COWS
>   	 * we completely stop queueing qios and wait
>   	 * for pending qios. Lock is for visability.
>   	 */
> -	spin_lock_irq(&qcow2->deferred_lock);
> -	qcow2->pause_submitting_qios = true;
> -	spin_unlock_irq(&qcow2->deferred_lock);
> +	spin_lock_irq(&top->deferred_lock);
> +	top->pause_submitting_qios = true;
> +	spin_unlock_irq(&top->deferred_lock);
>   	qcow2_inflight_ref_switch(tgt);
>   
>   	/* queue is stopped */
> -	spin_lock_irq(&qcow2->deferred_lock);
> +	spin_lock_irq(&top->deferred_lock);
>   	WARN_ON_ONCE(qcow2->backward_merge_in_process == set);
>   	qcow2->backward_merge_in_process = set;
> -	qcow2->pause_submitting_qios = false;
> -	list_splice_init(&qcow2->paused_qios, &list);
> -	spin_unlock_irq(&qcow2->deferred_lock);
> +	top->pause_submitting_qios = false;
> +	list_splice_init(&top->paused_qios, &list);
> +	spin_unlock_irq(&top->deferred_lock);
>   
>   	qcow2_submit_embedded_qios(tgt, &list);
>   }