Re: [Qemu-devel] [PATCH v12 1/2] block/backup: fix max_transfer handling for copy_range

[John Snow <jsnow@redhat.com>]

On 9/19/19 2:50 AM, Vladimir Sementsov-Ogievskiy wrote:
> 18.09.2019 22:57, John Snow wrote:
>>
>>
>> On 9/17/19 12:07 PM, Vladimir Sementsov-Ogievskiy wrote:
>>> Of course, QEMU_ALIGN_UP is a typo, it should be QEMU_ALIGN_DOWN, as we
>>> are trying to find aligned size which satisfy both source and target.
>>> Also, don't ignore too small max_transfer. In this case seems safer to
>>> disable copy_range.
>>>
>>> Fixes: 9ded4a0114968e
>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>>> ---
>>>   block/backup.c | 12 ++++++++----
>>>   1 file changed, 8 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/block/backup.c b/block/backup.c
>>> index 763f0d7ff6..d8fdbfadfe 100644
>>> --- a/block/backup.c
>>> +++ b/block/backup.c
>>> @@ -741,12 +741,16 @@ BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
>>>       job->cluster_size = cluster_size;
>>>       job->copy_bitmap = copy_bitmap;
>>>       copy_bitmap = NULL;
>>> -    job->use_copy_range = !compress; /* compression isn't supported for it */
>>>       job->copy_range_size = MIN_NON_ZERO(blk_get_max_transfer(job->common.blk),
>>>                                           blk_get_max_transfer(job->target));
>>> -    job->copy_range_size = MAX(job->cluster_size,
>>> -                               QEMU_ALIGN_UP(job->copy_range_size,
>>> -                                             job->cluster_size));
>>> +    job->copy_range_size = QEMU_ALIGN_DOWN(job->copy_range_size,
>>> +                                           job->cluster_size);
>>> +    /*
>>> +     * Compression is not supported for copy_range. Also, we don't want to
>>> +     * handle small max_transfer for copy_range (which currently don't
>>> +     * handle max_transfer at all).
>>> +     */
>>> +    job->use_copy_range = !compress && job->copy_range_size > 0;
>>>       /* Required permissions are already taken with target's blk_new() */
>>>       block_job_add_bdrv(&job->common, "target", target, 0, BLK_PERM_ALL,
>>>
>>
>> I'm clear on the alignment fix, I'm not clear on the comment about max_transfer and how it relates to copy_range_size being non-zero.
>>
>> "small max transfer" -- what happens when it's zero? we're apparently OK with a single cluster, but when it's zero, what happens?
> 
> if it zero it means that source or target requires max_transfer less than cluster_size. It seems not valid to call copy_range in this case.
> Still it's OK to use normal read/write, as they handle max_transfer internally in a loop (copy_range doesn't do it).
> 

oh, I'm ... sorry, I just didn't quite understand the comment.

You're just making sure copy_range after all of our checks is non-zero,
plain and simple. If max_transfer was *smaller than a job cluster*, we
might end up with a copy_range size that's zero, which is obviously...
not useful.

So, I might phrase "Also, we don't want to..." as:

"copy_range does not respect max_transfer, so we factor that in here. If
it's smaller than the job->cluster_size, we are unable to use copy_range."

Just a suggestion, though, so:

Reviewed-by: John Snow <jsnow@redhat.com>


(SHOULD copy_range respect max_transfer? I guess it would be quite
different -- it would only count things it had to fall back and actually
*transfer*, right? I suppose that because it can have that fallback we
need to accommodate it here in backup.c, hence this workaround clamp.)