01.10.2019 17:13, Max Reitz wrote: > On 01.10.19 16:00, Vladimir Sementsov-Ogievskiy wrote: >> 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; >> +} >> + > > Why would we skip filters explicitly added by the user? >
Why not? Otherwise migration of bitmaps will not work: we may have different set of filters on source and destination, and we still should map nodes with bitmaps automatically. I like John's idea of explicitly defined node mapping, but now we need simpler fix, not involving libvirt changes if possible. Hmm, or you mean that by this patch I touch not only migration but all users of bdrv_get_device_or_node_name? Than I can't predict all the consequences... Is it better to add .get_name_for_bitmaps_migration handler (with simpler name of course)? -- Best regards, Vladimir
