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

[Max Reitz <mreitz@redhat.com>]

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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 <vsementsov@virtuozzo.com>
>>> ---
>>>   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?

Max


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