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Re: [PATCH v3 09/21] quorum: Add QuorumChild.to_be_replaced

From: Max Reitz
Subject: Re: [PATCH v3 09/21] quorum: Add QuorumChild.to_be_replaced
Date: Thu, 6 Feb 2020 17:43:59 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1

On 06.02.20 16:51, Kevin Wolf wrote:
> Am 06.02.2020 um 16:21 hat Max Reitz geschrieben:
>> On 06.02.20 15:58, Kevin Wolf wrote:
>>> Am 06.02.2020 um 11:11 hat Max Reitz geschrieben:
>>>> On 05.02.20 16:38, Kevin Wolf wrote:
>>>>> Am 30.01.2020 um 22:44 hat Max Reitz geschrieben:
>>>>>> We will need this to verify that Quorum can let one of its children be
>>>>>> replaced without breaking anything else.
>>>>>> Signed-off-by: Max Reitz <address@hidden>
>>>>>> ---
>>>>>>  block/quorum.c | 25 +++++++++++++++++++++++++
>>>>>>  1 file changed, 25 insertions(+)
>>>>>> diff --git a/block/quorum.c b/block/quorum.c
>>>>>> index 59cd524502..6a7224c9e4 100644
>>>>>> --- a/block/quorum.c
>>>>>> +++ b/block/quorum.c
>>>>>> @@ -67,6 +67,13 @@ typedef struct QuorumVotes {
>>>>>>  typedef struct QuorumChild {
>>>>>>      BdrvChild *child;
>>>>>> +
>>>>>> +    /*
>>>>>> +     * If set, check whether this node can be replaced without any
>>>>>> +     * other parent noticing: Unshare CONSISTENT_READ, and take the
>>>>>> +     * WRITE permission.
>>>>>> +     */
>>>>>> +    bool to_be_replaced;
>>>>> I don't understand these permission changes. How does (preparing for)
>>>>> detaching a node from quorum make its content invalid?
>>>> It doesn’t, of course.  What we are preparing for is to replace it by
>>>> some other node with some other content.
>>>>> And why do we
>>>>> suddenly need WRITE permissions even if the quorum node is only used
>>>>> read-only?
>>>>> The comment is a bit unclear, too. "check whether" implies that both
>>>>> outcomes could be true, but it doesn't say what happens in either case.
>>>>> Is this really "make sure that"?
>>>> I think the comment is not only unclear, it is the problem.  (Well,
>>>> maybe the code is also.)
>>>> This series is about fixing at least some things about replacing nodes
>>>> by mirroring.  The original use cases this was introduced for was to fix
>>>> broken quorum children: The other children are still intact, so you read
>>>> from the quorum node and replace the broken child (which maybe shows
>>>> invalid data, or maybe just EIO) by the fixed mirror result.
>>>> Replacing that broken node by the fixed one changes the data that’s
>>>> visible on that node.
>>> Hm, yes, that's true. But I wonder if this is really something that the
>>> permission system must catch. Like other graph manipulations, it's
>>> essentially the user saying "trust me, I know what I'm doing, this node
>>> makes sense in this place".
>>> Because if you assume that the user could add a node with unsuitable
>>> content and you want to prevent this, where do we stop?
>>> blockdev-snapshot can insert a non-empty overlay, which would result in
>>> visible data change. Should we therefore only allow snapshots when
>>> shared writes are allowed? This doesn't work obviously.
>>> So I'm inclined to say that this is the user's responsibility and we
>>> don't have to jump through hoops to prevent every possible way that the
>>> user could mess up. (Which often also result in preventing legitimate
>>> cases like here a quorum of read-only nodes.)
>> Well, if you ask the question “where do we stop”, we also have to ask
>> the question “where do we start”.  If we say the user knows what they’re
>> doing, we might as well drop the whole can_replace infrastructure
>> altogether and just assume that you can replace any node by anything.
> Well, I don't actually know if that would be completely unreasonable.
> The idea was obviously to keep graph changes restricted to very specific
> cases to avoid nasty surprises like triggering latent bugs. Meanwhile we
> have quite a few more operations that allow changing the graph.
> So if preventing some cases gives us headaches and is probably more work
> than dealing with any bugs they might reveal, maybe preventing them is
> wrong.
> I'm just afraid that we might be overengineering this and waste time on
> things that we don't actually get much use from.

That’s why I’m asking.

>> If the WRITE permission is the problem, then I suppose we can drop that.
>>  Unsharing CONSISTENT_READ is bad enough that it effectively deters all
>> other parents anyway.
> WRITE is probably the more practical problem, though it's technically
> the correct one to take.
> CONSISTENT_READ is already a problem in theory because replacing a child
> node with different content doesn't even match its definition:
>     /**
>      * A user that has the "permission" of consistent reads is guaranteed that
>      * their view of the contents of the block device is complete and
>      * self-consistent, representing the contents of a disk at a specific
>      * point.
>      *
>      * For most block devices (including their backing files) this is true, 
> but
>      * the property cannot be maintained in a few situations like for
>      * intermediate nodes of a commit block job.
>      */
> Replacing an image with a different image means that the node represents
> the content of a different disk now, but it's probably still complete
> and self-consistent.

At any point in time yes, but not over the time span of the change.  The
definition doesn’t say that the node represents the contents of a disk
at a specific point, but the view from the parent.

I argue that that view is always over some period of time, so if you
suddenly switch out the whole disk, then it isn’t a self-consistent view.

Alternatively, we could of course also just forego the permission system
here altogether and just check that there are no other parents at all.
(Which is effectively the same as unsharing CONSISTENT_READ.)


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