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

[John Snow <jsnow@redhat.com>]

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?