Re: [PATCH for-5.0 v2 15/23] mirror: Prevent loops

[Max Reitz <mreitz@redhat.com>]

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On 29.11.19 14:55, Vladimir Sementsov-Ogievskiy wrote:
> 29.11.2019 16:46, Max Reitz wrote:
>> On 29.11.19 13:01, Vladimir Sementsov-Ogievskiy wrote:
>>> 11.11.2019 19:02, Max Reitz wrote:
>>>> While bdrv_replace_node() will not follow through with it, a specific
>>>> @replaces asks the mirror job to create a loop.
>>>>
>>>> For example, say both the source and the target share a child where the
>>>> source is a filter; by letting @replaces point to the common child, you
>>>> ask for a loop.
>>>>
>>>> Or if you use @replaces in drive-mirror with sync=none and
>>>> mode=absolute-paths, you generally ask for a loop (@replaces must point
>>>> to a child of the source, and sync=none makes the source the backing
>>>> file of the target after the job).
>>>>
>>>> bdrv_replace_node() will not create those loops, but by doing so, it
>>>> ignores the user-requested configuration, which is not ideally either.
>>>> (In the first example above, the target's child will remain what it was,
>>>> which may still be reasonable.  But in the second example, the target
>>>> will just not become a child of the source, which is precisely what was
>>>> requested with @replaces.)
>>>>
>>>> So prevent such configurations, both before the job, and before it
>>>> actually completes.
>>>>
>>>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>>>> ---
>>>>    block.c                   | 30 ++++++++++++++++++++++++
>>>>    block/mirror.c            | 19 +++++++++++++++-
>>>>    blockdev.c                | 48 ++++++++++++++++++++++++++++++++++++++-
>>>>    include/block/block_int.h |  3 +++
>>>>    4 files changed, 98 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/block.c b/block.c
>>>> index 0159f8e510..e3922a0474 100644
>>>> --- a/block.c
>>>> +++ b/block.c
>>>> @@ -6259,6 +6259,36 @@ out:
>>>>        return to_replace_bs;
>>>>    }
>>>>    
>>>> +/*
>>>> + * Return true iff @child is a (recursive) child of @parent, with at
>>>> + * least @min_level edges between them.
>>>> + *
>>>> + * (If @min_level == 0, return true if @child == @parent.  For
>>>> + * @min_level == 1, @child needs to be at least a real child; for
>>>> + * @min_level == 2, it needs to be at least a grand-child; and so on.)
>>>> + */
>>>> +bool bdrv_is_child_of(BlockDriverState *child, BlockDriverState *parent,
>>>> +                      int min_level)
>>>> +{
>>>> +    BdrvChild *c;
>>>> +
>>>> +    if (child == parent && min_level <= 0) {
>>>> +        return true;
>>>> +    }
>>>> +
>>>> +    if (!parent) {
>>>> +        return false;
>>>> +    }
>>>> +
>>>> +    QLIST_FOREACH(c, &parent->children, next) {
>>>> +        if (bdrv_is_child_of(child, c->bs, min_level - 1)) {
>>>> +            return true;
>>>> +        }
>>>> +    }
>>>> +
>>>> +    return false;
>>>> +}
>>>> +
>>>>    /**
>>>>     * Iterates through the list of runtime option keys that are said to
>>>>     * be "strong" for a BDS.  An option is called "strong" if it changes
>>>> diff --git a/block/mirror.c b/block/mirror.c
>>>> index 68a4404666..b258c7e98b 100644
>>>> --- a/block/mirror.c
>>>> +++ b/block/mirror.c
>>>> @@ -701,7 +701,24 @@ static int mirror_exit_common(Job *job)
>>>>             * there.
>>>>             */
>>>>            if (bdrv_recurse_can_replace(src, to_replace)) {
>>>> -            bdrv_replace_node(to_replace, target_bs, &local_err);
>>>> +            /*
>>>> +             * It is OK for @to_replace to be an immediate child of
>>>> +             * @target_bs, because that is what happens with
>>>> +             * drive-mirror sync=none mode=absolute-paths: target_bs's
>>>> +             * backing file will be the source node, which is also
>>>> +             * to_replace (by default).
>>>> +             * bdrv_replace_node() handles this case by not letting
>>>> +             * target_bs->backing point to itself, but to the source
>>>> +             * still.
>>>> +             */
>>>
>>> Hmm.. So, we want the following valid case:
>>>
>>> (other parents of source) ----> source = to_replace <--- backing --- target
>>>
>>> becomes
>>>
>>> (other parents of source) ----> target --- backing ---> source
>>>
>>> But it seems for me, that the following is not less valid:
>>>
>>> (other parents of source) ----> source = to_replace <--- backing --- X <--- backing --- target
>>>
>>> becomes
>>>
>>> (other parents of source) ----> target --- backing ---> X --- backing ---> source
>>
>> I think it is less valid.  The first case works with sync=none, because
>> target is initially empty and then you just copy all new data, so the
>> target keeps looking like the source.
>>
>> But in the second case, there are intermediate nodes that mean that
>> target may well not look like the source.
> 
> Maybe, it's valid if target node is a filter? Or, otherwise, it's backing is a filter,
> but this seems less useful.

The question to me is whether it’s really useful.  The thing is that
maybe bdrv_replace_node() can make sense of it.  But still, from the
user’s perspective, they kind of are asking for a loop whenever
to_replace is a child of target.  It just so happens that we must allow
one of these cases because it’s the default case for sync=none.

So I’d rather forbid all such cases, because it should be understandable
to users why...

Max


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