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

[Andrey Zhadchenko]

On 9/17/25 07:53, Pavel Tikhomirov wrote:
>> @@ -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?

I just didn't like to set pause_submitting_qios on non-top delta as it 
is completely pointless. But maybe you are right.

> 
> 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);
>>   }