Commit 9bd910e2 authored Jun 10, 2016 by Max Reitz

block: Allow replacement of a BDS by its overlay



change_parent_backing_link() asserts that the BDS to be replaced is not
used as a backing file. However, we may want to replace a BDS by its
overlay in which case that very link should not be redirected.

For instance, when doing a sync=none drive-mirror operation, we may have
the following BDS/BB forest before block job completion:

  target

  base <- source <- BlockBackend

During job completion, we want to establish the source BDS as the
target's backing node:

          target
            |
            v
  base <- source <- BlockBackend

This makes the target a valid replacement for the source:

          target <- BlockBackend
            |
            v
  base <- source

Without this modification to change_parent_backing_link() we have to
inject the target into the graph before the source is its backing node,
thus temporarily creating a wrong graph:

  target <- BlockBackend

  base <- source

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20160610185750.30956-2-mreitz@redhat.com
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

parent 87cd3d20

block.c

+15 −1

Original line number	Diff line number	Diff line
		@@ -2226,9 +2226,23 @@ void bdrv_close_all(void)
		static void change_parent_backing_link(BlockDriverState *from,
		BlockDriverState *to)
		{
		BdrvChild c, next;
		BdrvChild c, next, *to_c;

		QLIST_FOREACH_SAFE(c, &from->parents, next_parent, next) {
		if (c->role == &child_backing) {
		/* @from is generally not allowed to be a backing file, except for
		* when @to is the overlay. In that case, @from may not be replaced
		* by @to as @to's backing node. */
		QLIST_FOREACH(to_c, &to->children, next) {
		if (to_c == c) {
		break;
		}
		}
		if (to_c) {
		continue;
		}
		}

		assert(c->role != &child_backing);
		bdrv_ref(to);
		bdrv_replace_child(c, to);