Re: [RFC PATCH 0/2] Allow changing bs->file on reopen

From: Kevin Wolf <kwolf@redhat.com>
To: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Cc: Kashyap Chamarthy <kchamart@redhat.com>,
	Alberto Garcia <berto@igalia.com>,
	qemu-devel@nongnu.org, qemu-block@nongnu.org,
	Max Reitz <mreitz@redhat.com>
Subject: Re: [RFC PATCH 0/2] Allow changing bs->file on reopen
Date: Thu, 21 Jan 2021 11:52:17 +0100	[thread overview]
Message-ID: <20210121105217.GA5466@merkur.fritz.box> (raw)
In-Reply-To: <dafbbcb3-9d87-ab04-5413-eac551cdfe44@virtuozzo.com>

Am 18.01.2021 um 11:22 hat Vladimir Sementsov-Ogievskiy geschrieben:
> 15.01.2021 16:02, Alberto Garcia wrote:
> > Hi,
> > 
> > during the past months we talked about making x-blockdev-reopen stable
> > API, and one of the missing things was having support for changing
> > bs->file. See here for the discusssion (I can't find the message from
> > Kashyap that started the thread in the web archives):
> > 
> >     https://lists.gnu.org/archive/html/qemu-block/2020-10/msg00922.html
> > 
> > I was testing this and one of the problems that I found was that
> > removing a filter node using this command is tricky because of the
> > permission system, see here for details:
> > 
> >     https://lists.gnu.org/archive/html/qemu-block/2020-12/msg00092.html
> > 
> > The good news is that Vladimir posted a set of patches that changes
> > the way that permissions are updated on reopen:
> > 
> >     https://lists.gnu.org/archive/html/qemu-block/2020-11/msg00745.html
> > 
> > I was testing if this would be useful to solve the problem that I
> > mentioned earlier and it seems to be the case so I wrote a patch to
> > add support for changing bs->file, along with a couple of test cases.
> > 
> > This is still an RFC but you can see the idea.
> 
> Good idea and I glad to see that my patches help:)
> 
> Hmm, still, removing a filter which want to unshare WRITE even when
> doesn't have any parents will be a problem anyway, so we'll need a new
> command to drop filter with a logic like in bdrv_drop_filter in my
> series.
> 
> Or, we can introduce multiple reopen.. So that x-blockdev-reopen will
> take a list of BlockdevOptions, and do all modifications in one
> transaction. Than we'll be able to drop filter by transactional update
> of top node child and removing filter child link.

Internally, we already have reopen queues anyway, so it would make sense
to me to expose them externally and take a list of BlockdevOptions.
This way we should be able to do even complex changes of the graph where
adding some edges requires the removal of other edges in a single atomic
operation.

Kevin