Re: bitmap migration bug with -drive while block mirror runs

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

01.10.2019 3:09, John Snow wrote:
> Hi folks, I identified a problem with the migration code that Red Hat QE
> found and thought you'd like to see it:
> 
> https://bugzilla.redhat.com/show_bug.cgi?id=1652424#c20
> 
> Very, very briefly: drive-mirror inserts a filter node that changes what
> bdrv_get_device_or_node_name() returns, which causes a migration problem.
> 
> 
> Ignorant question #1: Can we multi-parent the filter node and
> source-node? It looks like at the moment both consider their only parent
> to be the block-job and don't have a link back to their parents otherwise.
> 
> 
> Otherwise: I have a lot of cloudy ideas on how to solve this, but
> ultimately what we want is to be able to find the "addressable" name for
> the node the bitmap is attached to, which would be the name of the first
> ancestor node that isn't a filter. (OR, the name of the block-backend
> above that node.)

Not the name of ancestor node, it will break mapping: it must be name of the
node itself or name of parent (may be through several filters) block-backend

> 
> A simple way to do this might be a "child_unfiltered" BdrvChild role
> that simply bypasses the filter that was inserted and serves no real
> purpose other than to allow the child to have a parent link and find who
> it's """real""" parent is.
> 
> Because of flushing, reopen, sync, drain &c &c &c I'm not sure how
> feasible this quick idea might be, though.
> 
> 
> - Corollary fix #1: call error_setg if the bitmap node name that's about
> to go over the wire is an autogenerated node: this is never correct!
> 
> (Why not? because the target is incapable of matching the node-name
> because they are randomly generated AND you cannot specify node-names
> with # prefixes as they are especially reserved!
> 
> (This raises a related problem: if you explicitly add bitmaps to nodes
> with autogenerated names, you will be unable to migrate them.))
> 
> --js
> 

What about the following:

diff --git a/block.c b/block.c
index 5944124845..6739c19be9 100644
--- a/block.c
+++ b/block.c
@@ -1009,8 +1009,20 @@ static void bdrv_inherited_options(int *child_flags, QDict *child_options,
      *child_flags = flags;
  }

+static const char *bdrv_child_get_name(BdrvChild *child)
+{
+    BlockDriverState *parent = child->opaque;
+
+    if (parent->drv && parent->drv->is_filter) {
+        return bdrv_get_parent_name(parent);
+    }
+
+    return NULL;
+}
+
  const BdrvChildRole child_file = {
      .parent_is_bds   = true,
+    .get_name        = bdrv_child_get_name,
      .get_parent_desc = bdrv_child_get_parent_desc,
      .inherit_options = bdrv_inherited_options,
      .drained_begin   = bdrv_child_cb_drained_begin,
@@ -1163,6 +1175,7 @@ static int bdrv_backing_update_filename(BdrvChild *c, BlockDriverState *base,

  const BdrvChildRole child_backing = {
      .parent_is_bds   = true,
+    .get_name        = bdrv_child_get_name,
      .get_parent_desc = bdrv_child_get_parent_desc,
      .attach          = bdrv_backing_attach,
      .detach          = bdrv_backing_detach,


-- 
Best regards,
Vladimir