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

[Eric Blake]

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.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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