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From: Wen Congyang <wency@cn.fujitsu.com>
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Date: Tue, 8 Sep 2015 17:10:22 +0800
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Subject: Re: [Qemu-devel] [Patch for-2.5 v2 3/6] Add new block driver
 interface to add/delete a BDS's child
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To: Eric Blake <eblake@redhat.com>, qemu devel <qemu-devel@nongnu.org>, Markus Armbruster <armbru@redhat.com>, Alberto Garcia <berto@igalia.com>, Stefan Hajnoczi <stefanha@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, zhanghailiang <zhang.zhanghailiang@huawei.com>, qemu block <qemu-block@nongnu.org>, Jiang Yunhong <yunhong.jiang@intel.com>, Dong Eddie <eddie.dong@intel.com>, "Dr. David Alan Gilbert" <dgilbert@redhat.com>, Gonglei <arei.gonglei@huawei.com>, Yang Hongyang <yanghy@cn.fujitsu.com>

On 09/01/2015 11:30 PM, Eric Blake wrote:
> On 08/31/2015 06:44 PM, Wen Congyang wrote:
> 
>>>
>>>> + * Hot add/remove a BDS's child. So the user can take a child offline when
>>>> + * it is broken and take a new child online
>>>> + */
>>>> +void bdrv_add_child(BlockDriverState *bs, QDict *options, Error **errp)
>>>> +{
>>>> +
>>>> +    if (!bs->drv || !bs->drv->bdrv_add_child) {
>>>> +        error_setg(errp, "The BDS %s doesn't support adding a child",
>>>> +                   bdrv_get_device_or_node_name(bs));
>>>> +        return;
>>>> +    }
>>>> +
>>>> +    bs->drv->bdrv_add_child(bs, options, errp);
>>>
>>> Should this also check that bs is not already a child of something?  Or
>>> a bit looser, we may want to allow a BDS to be a child of multiple trees
>>> (a common shared backing file), but we still definitely don't want to
>>> allow nonsensical loops such as trying to make a BDS be hot-added as its
>>> own child.
>>>
>>
>> hot-added BDS is a new BDS, but it is OK to check it here. I will update it
>> in the next version.
> 
> Design-wise, I think we really want to have the add-child operation be
> handed a pre-opened BDS, rather than the options dictionary to open the
> BDS itself.  That is, we should use the existing blockdev-add (and
> enhance it to support everything) to open the BDS, and then this command
> should just attach that BDS as the new child (which is why it IS
> important that we validate that the new BDS being added doesn't create
> an invalid loop).
> 

How to check it? The parent BDS can get all children. But the child doesn't
know if it is some BDS's child.

Thanks
Wen Congyang