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Date: Fri, 16 Nov 2018 16:18:34 +0100
From: Kevin Wolf <kwolf@redhat.com>
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Subject: Re: [Qemu-devel] KVM Forum block no[td]es
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To: Alberto Garcia <berto@igalia.com>
Cc: Max Reitz <mreitz@redhat.com>, Qemu-block <qemu-block@nongnu.org>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, Markus Armbruster <armbru@redhat.com>, "Denis V. Lunev" <den@openvz.org>, Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

Am 16.11.2018 um 16:03 hat Alberto Garcia geschrieben:
> > I don't think anything needs a way to generally block graph changes
> > around some node.  We only need to prevent changes to very specific
> > sets of edges.  This is something that the permission system just
> > cannot do.
> 
> But what would you do then?

I agree with you mostly in that I think that most problems that Max
mentioned aren't readl. The only real problem I see with GRAPH_MOD as a
permission on the node level is this overblocking - but that's bad
enough that I feel using permissions to block changes to a whole node is
not a good solution.

So what's the alternative? Max had a possible solution in the first
email in this thread:

> - A property of BdrvChild that can be set by a non-parent seems more
>   feasible, e.g. a counter where changing the child is possible only
>   if the counter is 0.  This also actually makes sense in what it
>   means.

The commit job would increment BdrvChild.block_change (or whatever we
would call it) for all bs->backing edges in the subchain.

Kevin