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

[Kevin Wolf]

Am 01.10.2019 um 16:34 hat Max Reitz geschrieben:
> On 01.10.19 16:27, Vladimir Sementsov-Ogievskiy wrote:
> > 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.
> 
> Why would we have a different set of explicitly added filters on source
> and destination and allow them to be automatically changed during
> migration?  Shouldn’t users only change them pre or post migration?

We never made a requirement that the backend must be the same on the
source and the destination. Basically, migration copies the state of
frontends and the user is responsible for having these frontends created
and connected to the right backends on the destination.

Using different paths on the destination is a very obvious requirement
for block devices. It's less obvious for the graph structure, but I
don't see a reason why it couldn't change on migration. Say we were
using local storage on the source, but now we did storage migration to
some network storage, access to which should be throttled.

Kevin

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