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

[Vladimir Sementsov-Ogievskiy]

16.01.2019 16:05, Max Reitz wrote:
> On 14.01.19 15:48, Vladimir Sementsov-Ogievskiy wrote:
>> 14.01.2019 17:13, Max Reitz wrote:
>>> On 14.01.19 15:01, Vladimir Sementsov-Ogievskiy wrote:
>>>> 14.01.2019 16:10, Max Reitz wrote:
>>>>> On 29.12.18 13:20, Vladimir Sementsov-Ogievskiy wrote:
>>>>>> Simplify backup_incremental_init_copy_bitmap using the function
>>>>>> bdrv_dirty_bitmap_next_dirty_area.
>>>>>>
>>>>>> Note: move to job->len instead of bitmap size: it should not matter but
>>>>>> less code.
>>>>>>
>>>>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
>>>>>> ---
>>>>>>     block/backup.c | 40 ++++++++++++----------------------------
>>>>>>     1 file changed, 12 insertions(+), 28 deletions(-)
>>>>>
>>>>> Overall: What is this function even supposed to do?  To me, it looks
>>>>> like it marks all areas in job->copy_bitmap dirty that are dirty in
>>>>> job->sync_bitmap.
>>>>>
>>>>> If so, wouldn't just replacing this by hbitmap_merge() simplify things
>>>>> further?
>>>>>
>>>>>> diff --git a/block/backup.c b/block/backup.c
>>>>>> index 435414e964..fbe7ce19e1 100644
>>>>>> --- a/block/backup.c
>>>>>> +++ b/block/backup.c
>>>>>> @@ -406,43 +406,27 @@ static int coroutine_fn 
>>>>>> backup_run_incremental(BackupBlockJob *job)
>>>>>
>>>>> [...]
>>>>>
>>>>>> +    while (bdrv_dirty_bitmap_next_dirty_area(job->sync_bitmap,
>>>>>> +                                             &offset, &bytes))
>>>>>> +    {
>>>>>> +        uint64_t cluster = offset / job->cluster_size;
>>>>>> +        uint64_t last_cluster = (offset + bytes) / job->cluster_size;
>>>>>>     
>>>>>> -        next_cluster = DIV_ROUND_UP(offset, job->cluster_size);
>>>>>> -        hbitmap_set(job->copy_bitmap, cluster, next_cluster - cluster);
>>>>>> -        if (next_cluster >= end) {
>>>>>> +        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.

-- 
Best regards,
Vladimir