Re: [Qemu-devel] [PATCH v5 01/11] block/backup: simplify backup_incremental_init_copy_bitmap

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

23.01.2019 17:36, Eric Blake wrote:
> On 1/23/19 2:20 AM, Vladimir Sementsov-Ogievskiy wrote:
> 
>>>>>>>> +        hbitmap_set(job->copy_bitmap, cluster, last_cluster - cluster + 1);
>>>>>>>
>>>>>>> Why the +1?  Shouldn't the division for last_cluster round up instead?
>>>>>>>
>>>>>>>> +
>>>>>>>> +        offset = (last_cluster + 1) * job->cluster_size;
>>>>>>>
>>>>>>> Same here.
>>>>>>
>>>>>> last cluster is not "end", but it's last dirty cluster. so number of dirty clusters is last_cluster - cluster + 1, and next offset is calculated through +1 too.
>>>>>>
>>>>>> If I round up division result, I'll get last for most cases, but "end" (next after the last), for the case when offset % job->cluster_size == 0, so, how to use it?
>>>>>
>>>>> Doesn't bdrv_dirty_bitmap_next_dirty_area() return a range [offset,
>>>>> offset + bytes), i.e. where "offset + bytes" is the first clean offset?
>>>>
>>>> oops, you are right. then I need
>>>> uint64_t last_cluster = (offset + bytes - 1) / job->cluster_size;
>>>
>>> That, or you just use a rounding up division and rename it from
>>> last_cluster to end_cluster or first_clean_cluster or something (and
>>> subsequently drop the +1s).
>>
>> This will not work, as ((offset + bytes) / job->cluster_size) is not first clean cluster
>> or end cluster. It's a cluster, where is first clean bit located, but it may have dirty
>> bits too (or, may not).
>>
>> So, to rewrite based on end_cluster, it should be calculated as
>>
>> (offset + bytes - 1) / job->cluster_size + 1
>>
>> and, I'm going to do so, one "+1" instead of two, and, may be, a bit more understandable.
> 
> Better is to use the macros in osdep.h, such as QEMU_ALIGN_UP/DOWN, to
> make the intent of the code easier to read than having to closely check
> which operation is being performed to see if it makes sense in context.
> 

Yes, thanks. Exactly:
uint64_t end_cluster = DIV_ROUND_UP(offset + bytes, job->cluster_size);


-- 
Best regards,
Vladimir