Re: [PATCH 02/18] fuse: Allow exporting BDSs via FUSE

[Kevin Wolf <kwolf@redhat.com>]

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Am 20.12.2019 um 11:48 hat Max Reitz geschrieben:
> On 20.12.19 11:26, Kevin Wolf wrote:
> > Am 19.12.2019 um 15:38 hat Max Reitz geschrieben:
> >> fuse-export-add allows mounting block graph nodes via FUSE on some
> >> existing regular file.  That file should then appears like a raw disk
> >> image, and accesses to it result in accesses to the exported BDS.
> >>
> >> Right now, we only set up the mount point and tear all mount points down
> >> in bdrv_close_all().  We do not implement any access functions, so
> >> accessing the mount point only results in errors.  This will be
> >> addressed by a followup patch.
> >>
> >> The set of exported nodes is kept in a hash table so we can later add a
> >> fuse-export-remove that allows unmounting.
> >>
> >> Signed-off-by: Max Reitz <mreitz@redhat.com>
> > 
> >> diff --git a/qapi/block.json b/qapi/block.json
> >> index 145c268bb6..03f8d1b537 100644
> >> --- a/qapi/block.json
> >> +++ b/qapi/block.json
> >> @@ -317,6 +317,29 @@
> >>  ##
> >>  { 'command': 'nbd-server-stop' }
> >>  
> >> +##
> >> +# @fuse-export-add:
> >> +#
> >> +# Exports a block graph node on some (file) mountpoint as a raw image.
> >> +#
> >> +# @node-name: Node to be exported
> >> +#
> >> +# @mountpoint: Path on which to export the block device via FUSE.
> >> +#              This must point to an existing regular file.
> >> +#
> >> +# @writable: Whether clients should be able to write to the block
> >> +#            device via the FUSE export. (default: false)
> >> +#
> >> +# Since: 5.0
> >> +##
> >> +{ 'command': 'fuse-export-add',
> >> +  'data': {
> >> +      'node-name': 'str',
> >> +      'mountpoint': 'str',
> >> +      '*writable': 'bool'
> >> +  },
> >> +  'if': 'defined(CONFIG_FUSE)' }
> > 
> > Can this use a BlockExport union from the start like I'm introducing in
> > the storage daemon series, together with a generic block-export-add?
> 
> Hm, you mean still adding a FuseExport structure that would be part of
> BlockExport and then dropping fuse-export-add in favor of a
> block-export-add that we want anyway?

Yes.

> > It also looks like node-name and writable should be part of the common
> > base of BlockExport.
> 
> node-name definitely, I’m not so sure about writable.  Or, to be more
> precise, I think that if we want writable to be in the base, we also
> want growable to be there: Both are primarily options for the
> BlockBackend that the exports use.
> 
> But both of course also need to be supported by the export
> implementation.  nbd can make its BB growable all it wants, but that
> doesn’t make it work.

Right. Pragmatically, I think exports are very like to support writable,
but probably rather unlikely to support growable. So I do think there
would be a point for making writable part of the common base, but not
growable.

> So if we kept writable and growable in the common base, then the schema
> would give no information about what exports actually support them.
> 
> On one hand, I don’t know whether it’s important to have this
> information in a static form, or whether it’s sufficient to learn at
> runtime.
> 
> On the other, I don’t know whether it’s important to have those fields
> in the base or not.  Would it make a difference on the wire?

Not for the command itself, so I think we're free to change it later. It
might make a difference for introspection, though, not sure. Markus?

Having it in the base might allow us to remove some duplication in the
code. Probably not much, though, so not too important.

> > Unfortunately this would mean that I can't use the
> > same BlockExportNbd for the existing nbd-server-add command any more. I
> > guess I could somehow get a shared base type for both, though.
> 
> Hm.  This sounds like you want to make it your problem.  Can I take that
> to mean that you want to implement block-export-add and I can wait with
> v2 until that’s done? :-)

The NBD integration, yes. I already added the BlockExport type to my
patches, too, but I expect you would beat me to it. I'm not currently
planning to write a block-export-add because it doesn't add anything new
for the storage daemon, so FuseExport and the command this is your part.
The type currently only exists for --export.

Kevin


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