* Max Reitz (mre...@redhat.com) wrote: > On 14.05.20 10:42, Dr. David Alan Gilbert wrote: > > * Max Reitz (mre...@redhat.com) wrote: > > > > <snip> > > > >> +void qmp_migrate_set_bitmap_node_mapping(MigrationBlockNodeMappingList > >> *mapping, > >> + Error **errp) > >> +{ > >> + QDict *in_mapping = qdict_new(); > >> + QDict *out_mapping = qdict_new(); > >> + > >> + for (; mapping; mapping = mapping->next) { > >> + MigrationBlockNodeMapping *entry = mapping->value; > >> + > >> + if (qdict_haskey(out_mapping, entry->node_name)) { > >> + error_setg(errp, "Cannot map node name '%s' twice", > >> + entry->node_name); > >> + goto fail; > >> + } > > > > I'm not too clear exactly which case this is protecting against; > > I think that's protecting against mapping > > > > 'src1'->'dst1' and 'src1'->'dst2' > > which is a good check.s (or maybe it's checking against dst2 twice?) > > This one is against mapping src1 twice. Both checks together check that > it’s a one-to-one bijective mapping. > > The technical reason why it needs to be one-to-one is because we base > two QDicts off of it, so the inverse mapping needs to work. > > > What about cases where there is no mapping - e.g. imagine > > that we have b1/b2 on the source and b2/b3 on the dest; now > > if we add just a mapping: > > > > b1->b2 > > > > then we end up with: > > > > b1 -> b2 > > b2 -> b2 (non-mapped) > > b3 > > > > so we have a clash there - are we protected against that? > > Oh, no, we aren’t. That wasn’t intentional. However, I’m not sure how > we’d protect against it. We can’t check it in > qmp_migrate_set_bitmap_node_mapping(), because we don’t know yet which > nodes will exist at the time of migration, and which of those will have > bitmaps. > > So we’d need to check it as part of the migration process (by looking up > any unmapped entries that default to the identity mapping in the > respective reverse mapping, to see whether anything maps to the same name).
Yeh a once through check of all the nodes at the start of the migration would probably fix it. > OTOH, Vladimir proposed adding a parameter to > migrate-set-bitmap-node-mapping that would make migration fail if any > bitmaps should be migrated off of unmapped nodes, or if any incoming > alias is unmapped (instead of defaulting to the identity mapping). If > we just make that the only behavior, then we wouldn’t have a problem > with that at all, because all unmapped nodes would always throw an error. Yeh that would force you to put a full mapping table in place. > (And on the third hand, I wonder whether we should actually allow > migrating bitmaps from multiple nodes to a single one, but I suppose > that would require two separate commands, one for incoming and one for > outgoing...) Wouldn't that get very messy? Dave > Max > -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK