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


From: Kevin Wolf
Subject: Re: [PATCH v3 09/21] quorum: Add QuorumChild.to_be_replaced
Date: Thu, 6 Feb 2020 18:41:49 +0100
User-agent: Mutt/1.12.1 (2019-06-15)

Am 06.02.2020 um 18:06 hat Max Reitz geschrieben:
> On 06.02.20 17:57, Kevin Wolf wrote:
> > Am 06.02.2020 um 17:43 hat Max Reitz geschrieben:
> >> 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.
> > 
> > Did I answer your question sufficiently then?
> 
> No, because “I’m afraid” is a sentiment I fully share, but it doesn’t
> answer the question whether we are indeed overengineering this or not. :-)

Well, I guess I can only answer this after seeing the bug reports we
would get after removing the check. :-)

> I suppose my stance now is “This is probably overengineered, but now we
> might as well roll with it”.

Your choice. I'm not opposed to anything that feels like it makes sense.

> >>>> 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.
> >>>      */
> >>>     BLK_PERM_CONSISTENT_READ    = 0x01,
> >>>
> >>> 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.
> > 
> > I think your theory that it's over some period of time conflicts with
> > the documentation that says "at a specific point".
> 
> I’d rather not get into a deeper discussion on what CONSISTENT_READ
> means again... :-/
> 
> I always feel like if you really take only a single point in time, then
> anything could be some hypothetical disk.
> 
> So to me, unsharing CONSISTENT_READ effectively just means “Don’t touch
> this, you don’t want to”.

The difference is that with the replace operation we aren't talking
about hypothetical corrupted disks (like we would get when accessing
intermediate nodes of the commit job), but about two actual disk images
that are both valid, though different.

But yes, maybe we should avoid this discussion...

(I mean, what it *really* means is "this is not an intermediate node
of commit". ;-))

> >> 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.)
> > 
> > This would sidestep all of the artificial permission twiddling, which
> > sounds good.
> > 
> > It would probably also needlessly restrict the allowed use cases,
> 
> Only in theory, though, because in practice basically everything useful
> takes CONSISTENT_READ anyway.

Oh, compared to taking WRITE and unsharing CONSISTENT_READ it's probably
not more restrictice. I was comparing with the case that drops the
checks altogether.

> > but
> > then, who cares about nodes with multiple parents, one of which is a
> > quorum node?
> > 
> > So I guess I would be fine with either checking that there are no
> > parents or maybe even just dropping the check completely.
> 
> OK, I’ll check the parent list then.  (Except it must be exactly one
> parent, namely Quorum.)

Fine with me.

Kevin

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