mirror: optimize dirty bitmap filling in mirror_run a bit

There is no need to scan allocation tables if we have mark_all_dirty flag
set. Just mark it all dirty.
Signed-off-by: NDenis V. Lunev <den@openvz.org>
Reviewed-by: Vladimir Sementsov-Ogievskiy<vsementsov@virtuozzo.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NJohn Snow <jsnow@redhat.com>
Reviewed-by: NFam Zheng <famz@redhat.com>
Message-id: 1468503209-19498-7-git-send-email-den@openvz.org
CC: Stefan Hajnoczi <stefanha@redhat.com>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Max Reitz <mreitz@redhat.com>
CC: Jeff Cody <jcody@redhat.com>
Signed-off-by: NJeff Cody <jcody@redhat.com>

block/mirror.c

@@ -533,11 +533,15 @@ static int coroutine_fn mirror_dirty_init(MirrorBlockJob *s)
     BlockDriverState *base = s->base;
     BlockDriverState *bs = blk_bs(s->common.blk);
     BlockDriverState *target_bs = blk_bs(s->target);
-    bool mark_all_dirty = base == NULL && !bdrv_has_zero_init(target_bs);
     int ret, n;
 
     end = s->bdev_length / BDRV_SECTOR_SIZE;
 
+    if (base == NULL && !bdrv_has_zero_init(target_bs)) {
+        bdrv_set_dirty_bitmap(s->dirty_bitmap, 0, end);
+        return 0;
+    }
+
     /* First part, loop on the sectors and initialize the dirty bitmap.  */
     for (sector_num = 0; sector_num < end; ) {
         /* Just to make sure we are not exceeding int limit. */
@@ -556,7 +560,7 @@ static int coroutine_fn mirror_dirty_init(MirrorBlockJob *s)
         }
 
         assert(n > 0);
-        if (ret == 1 || mark_all_dirty) {
+        if (ret == 1) {
             bdrv_set_dirty_bitmap(s->dirty_bitmap, sector_num, n);
         }
         sector_num += n;