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

[Vladimir Sementsov-Ogievskiy]

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.

> These patches apply on top of Vladimir's branch:
>
> git: https://src.openvz.org/scm/~vsementsov/qemu.git
> tag: up-block-topologic-perm-v2
>
> Opinions are very welcome!
>
> Berto
>
> Alberto Garcia (2):
>    block: Allow changing bs->file on reopen
>    iotests: Update 245 to support replacing files with x-blockdev-reopen
>
>   include/block/block.h      |  1 +
>   block.c                    | 61 ++++++++++++++++++++++++++++++++++++++
>   tests/qemu-iotests/245     | 61 +++++++++++++++++++++++++++++++++++---
>   tests/qemu-iotests/245.out |  4 +--
>   4 files changed, 121 insertions(+), 6 deletions(-)


--
Best regards,
Vladimir