block/backup: fix max_transfer handling for copy_range

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: 9ded4a0
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20190920142056.12778-2-vsementsov@virtuozzo.com
Signed-off-by: Max Reitz <mreitz@redhat.com>

block/backup.c

@@ -741,12 +741,19 @@ 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);
+    /*
+     * Set use_copy_range, consider the following:
+     * 1. Compression is not supported for copy_range.
+     * 2. copy_range does not respect max_transfer (it's a TODO), so we factor
+     *    that in here. If max_transfer is smaller than the job->cluster_size,
+     *    we do not use copy_range (in that case it's zero after aligning down
+     *    above).
+     */
+    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,