block: improve block_dirty_bitmap_merge(): don't allocate extra bitmap

We don't need extra bitmap. All we need is to backup the original
bitmap when we do first merge. So, drop extra temporary bitmap and work
directly with target and backup.

Still to keep old semantics, that on failure target is unchanged and
user don't need to restore, we need a local_backup variable and do
restore ourselves on failure path.

Signed-off-by: Vladimir Sementsov-Ogievskiy <v.sementsov-og@mail.ru>
Message-Id: <20220517111206.23585-3-v.sementsov-og@mail.ru>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/monitor/bitmap-qmp-cmds.c

@@ -261,8 +261,9 @@ BdrvDirtyBitmap *block_dirty_bitmap_merge(const char *node, const char *target,
                                           HBitmap **backup, Error **errp)
 {
     BlockDriverState *bs;
-    BdrvDirtyBitmap *dst, *src, *anon;
+    BdrvDirtyBitmap *dst, *src;
     BlockDirtyBitmapOrStrList *lst;
+    HBitmap *local_backup = NULL;
 
     GLOBAL_STATE_CODE();
 
@@ -271,12 +272,6 @@ BdrvDirtyBitmap *block_dirty_bitmap_merge(const char *node, const char *target,
         return NULL;
     }
 
-    anon = bdrv_create_dirty_bitmap(bs, bdrv_dirty_bitmap_granularity(dst),
-                                    NULL, errp);
-    if (!anon) {
-        return NULL;
-    }
-
     for (lst = bms; lst; lst = lst->next) {
         switch (lst->value->type) {
             const char *name, *node;
@@ -285,8 +280,7 @@ BdrvDirtyBitmap *block_dirty_bitmap_merge(const char *node, const char *target,
             src = bdrv_find_dirty_bitmap(bs, name);
             if (!src) {
                 error_setg(errp, "Dirty bitmap '%s' not found", name);
-                dst = NULL;
-                goto out;
+                goto fail;
             }
             break;
         case QTYPE_QDICT:
@@ -294,29 +288,36 @@ BdrvDirtyBitmap *block_dirty_bitmap_merge(const char *node, const char *target,
             name = lst->value->u.external.name;
             src = block_dirty_bitmap_lookup(node, name, NULL, errp);
             if (!src) {
-                dst = NULL;
-                goto out;
+                goto fail;
             }
             break;
         default:
             abort();
         }
 
-        if (!bdrv_merge_dirty_bitmap(anon, src, NULL, errp)) {
-            dst = NULL;
-            goto out;
+        /* We do backup only for first merge operation */
+        if (!bdrv_merge_dirty_bitmap(dst, src,
+                                     local_backup ? NULL : &local_backup,
+                                     errp))
+        {
+            goto fail;
         }
     }
 
-    /* Merge into dst; dst is unchanged on failure. */
-    if (!bdrv_merge_dirty_bitmap(dst, anon, backup, errp)) {
-        dst = NULL;
-        goto out;
+    if (backup) {
+        *backup = local_backup;
+    } else {
+        hbitmap_free(local_backup);
     }
 
- out:
-    bdrv_release_dirty_bitmap(anon);
     return dst;
+
+fail:
+    if (local_backup) {
+        bdrv_restore_dirty_bitmap(dst, local_backup);
+    }
+
+    return NULL;
 }
 
 void qmp_block_dirty_bitmap_merge(const char *node, const char *target,