diff --git a/block.c b/block.c
index 1eee394..f40b014 100644
--- a/block.c
+++ b/block.c
@@ -113,6 +113,7 @@ static void bdrv_set_dirty(BlockDriverState *bs, int64_t cur_sector,
                            int nr_sectors);
 static void bdrv_reset_dirty(BlockDriverState *bs, int64_t cur_sector,
                              int nr_sectors);
+static void bdrv_dirty_bitmap_truncate(BlockDriverState *bs);
 /* If non-zero, use only whitelisted block drivers */
 static int use_bdrv_whitelist;
 
@@ -3543,6 +3544,7 @@ int bdrv_truncate(BlockDriverState *bs, int64_t offset)
     ret = drv->bdrv_truncate(bs, offset);
     if (ret == 0) {
         ret = refresh_total_sectors(bs, offset >> BDRV_SECTOR_BITS);
+        bdrv_dirty_bitmap_truncate(bs);
         if (bs->blk) {
             blk_dev_resize_cb(bs->blk);
         }
@@ -5562,6 +5564,22 @@ BdrvDirtyBitmap *bdrv_reclaim_dirty_bitmap(BlockDriverState *bs,
     return parent;
 }
 
+/**
+ * Truncates _all_ bitmaps attached to a BDS.
+ */
+static void bdrv_dirty_bitmap_truncate(BlockDriverState *bs)
+{
+    BdrvDirtyBitmap *bitmap;
+    uint64_t size = bdrv_nb_sectors(bs);
+
+    QLIST_FOREACH(bitmap, &bs->dirty_bitmaps, list) {
+        if (bdrv_dirty_bitmap_frozen(bitmap)) {
+            continue;
+        }
+        hbitmap_truncate(bitmap->bitmap, size);
+    }
+}
+
 void bdrv_release_dirty_bitmap(BlockDriverState *bs, BdrvDirtyBitmap *bitmap)
 {
     BdrvDirtyBitmap *bm, *next;
diff --git a/include/qemu/hbitmap.h b/include/qemu/hbitmap.h
index c19c1cb..a75157e 100644
--- a/include/qemu/hbitmap.h
+++ b/include/qemu/hbitmap.h
@@ -65,6 +65,16 @@ struct HBitmapIter {
 HBitmap *hbitmap_alloc(uint64_t size, int granularity);
 
 /**
+ * hbitmap_truncate:
+ * @hb: The bitmap to change the size of.
+ * @size: The number of elements to change the bitmap to accommodate.
+ *
+ * truncate or grow an existing bitmap to accommodate a new number of elements.
+ * This may invalidate existing HBitmapIterators.
+ */
+void hbitmap_truncate(HBitmap *hb, uint64_t size);
+
+/**
  * hbitmap_merge:
  * @a: The bitmap to store the result in.
  * @b: The bitmap to merge into @a.
diff --git a/util/hbitmap.c b/util/hbitmap.c
index ba11fd3..4505ef7 100644
--- a/util/hbitmap.c
+++ b/util/hbitmap.c
@@ -400,6 +400,58 @@ HBitmap *hbitmap_alloc(uint64_t size, int granularity)
     return hb;
 }
 
+void hbitmap_truncate(HBitmap *hb, uint64_t size)
+{
+    bool shrink;
+    unsigned i;
+    uint64_t num_elements = size;
+    uint64_t old;
+
+    /* Size comes in as logical elements, adjust for granularity. */
+    size = (size + (1ULL << hb->granularity) - 1) >> hb->granularity;
+    assert(size <= ((uint64_t)1 << HBITMAP_LOG_MAX_SIZE));
+    shrink = size < hb->size;
+
+    /* bit sizes are identical; nothing to do. */
+    if (size == hb->size) {
+        return;
+    }
+
+    /* If we're losing bits, let's clear those bits before we invalidate all of
+     * our invariants. This helps keep the bitcount consistent, and will prevent
+     * us from carrying around garbage bits beyond the end of the map.
+     *
+     * Because clearing bits past the end of map might reset bits we care about
+     * within the array, record the current value of the last bit we're keeping.
+     */
+    if (shrink) {
+        bool set = hbitmap_get(hb, num_elements - 1);
+        uint64_t fix_count = (hb->size << hb->granularity) - num_elements;
+
+        assert(fix_count);
+        hbitmap_reset(hb, num_elements, fix_count);
+        if (set) {
+            hbitmap_set(hb, num_elements - 1, 1);
+        }
+    }
+
+    hb->size = size;
+    for (i = HBITMAP_LEVELS; i-- > 0; ) {
+        size = MAX(BITS_TO_LONGS(size), 1);
+        if (hb->sizes[i] == size) {
+            break;
+        }
+        old = hb->sizes[i];
+        hb->sizes[i] = size;
+        hb->levels[i] = g_realloc(hb->levels[i], size * sizeof(unsigned long));
+        if (!shrink) {
+            memset(&hb->levels[i][old], 0x00,
+                   (size - old) * sizeof(*hb->levels[i]));
+        }
+    }
+}
+
+
 /**
  * Given HBitmaps A and B, let A := A (BITOR) B.
  * Bitmap B will not be modified.
