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

[Kevin Wolf]

Am 01.10.2019 um 10:57 hat Vladimir Sementsov-Ogievskiy geschrieben:
> 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.)
> 
> 
> Better would be to migrate by node-name only.. But am I right that
> node-names are different on source and destination? Or this situation
> changed?

Traditionally, I think migration assumes that frontends (guest devices)
must match exactly, but backends may and usually will differ.

Of course, dirty bitmaps are a backend feature that isn't really related
to guest devices, so this doesn't really work out any more in your case.
BlockBackend names are unusable for this purpose (especially as we're
moving towards anonymous BlockBackends everywhere), which I guess
essentially means node-name is the only option left.

Is bitmap migration something that must be enabled explicitly or does
it happen automatically? If it's explicit, then making an additional
requirement (matching node-names) shouldn't be a problem.

> > 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.))
> > 
> 
> In other words, we need a well defined way to match nodes on source and 
> destination,
> keeping in mind filters, to migrate bitmaps correctly.
> 
> Hm, did you thought about bitmaps in filters? It's not a problem to create 
> bitmap in
> mirror-top filter during mirror job:)
> 
> Or what about bitmaps in Quorum children? Or what about bitmap in qcow2 file 
> child bs?
> 
> If node-names are different on source and destination, what is the same? Top 
> blk name
> and bdrv-children names (I recently saw Max's idea to check node "path" in 
> iotest).

blk_name has to be assumed to be "". The BdrvChild path changes when
filters are inserted (and inserting filters on the destination that
aren't present on the source, or vice versa, sounds like something that
should just work).

So both parts of this are not great ways for addressing nodes.

> So, actually node is migration-addressable, if path 
> <blk-name>/root[/child-name] to the
> defines this node directly (we must not have children with same name for some 
> node in
> the path).
> 
> And I think it's a correct way to define node in migration stream - by path.

I'm afraid node-name is the only thing that could possibly work reliably
for identifying nodes.

Kevin