[Virtio-fs] Ways to uniquely and persistently identify nodes

[Max Reitz]

On 15.01.20 15:58, Miklos Szeredi wrote:
> On Wed, Jan 15, 2020 at 1:51 PM Max Reitz <mreitz at redhat.com> wrote:
>>
>> On 15.01.20 12:51, Miklos Szeredi wrote:
>>> On Wed, Jan 15, 2020 at 11:10 AM Miklos Szeredi <mszeredi at redhat.com> wrote:
>>>
>>>> New operations needed:
>>>>
>>>> LOOKUP_WITH_HANDLE: same as LOOKUP, but returns handle in addition to
>>>> fuse_ino_t and attributes.
>>>> LOOKUP_BY_HANDLE: same as LOOKUP, but gets a handle as input.
>>>
>>> And that can be reduced further to a single extended LOOKUP_HANDLE,
>>> which takes a handle as input and returns a handle in addition to the
>>> usual stuff.  The lookup-by-handle case can be done with a special
>>> name.  Currently "." is used for this purpose in the fuse protocol,
>>> which is a bit confusing since it has nothing to do the "." directory
>>> entry, but at least we don't have to introduce a new concept for this.
>>
>> I’m afraid I don’t quite understand.  What handle would LOOKUP_HANDLE
>> take as input when I want to open a new file (as in, LOOKUP_WITH_HANDLE)?
> 
> For example open("/foo/bar", ...) would do:
> 
> lookup_handle($ROOT_HANDLE, "foo") -> { FOO_HANDLE, FOO_NODEID }
> lookup_handle($FOO_HANDLE, "bar") -> { BAR_HANDLE, BAR_NODEID }
> open($BAR_NODEID, ...)

OK.  It seems a bit weird to me because if all other commands used a
fuse_ino_t to identify nodes, I think lookup should be no different.

Sure, we’d only need one new command instead of two, but naively just
adding two commands doesn’t seem fundamentally more difficult to me.

Specifically, what seems a bit problematic to me is that for virtiofsd
we’d probably want fuse_ino_t to stay as it is: An index into a vector.
 We probably don’t want to store the handle in virtiofsd at all.

So whenever a handle comes in, we’d first need to open it with
open_by_handle_at().  That’s OK if handles only come in through
lookup_by_handle(), i.e. when there is no valid fuse_ino_t value anyway.
 But it seems a bit cumbersome if all lookups are based on handles.

OTOH, we could keep the handles around, but that would probably require
a hash map with the handle as a key (so we can let lookup() find the
existing fd without having to open a new one).  Doable, sure, but I
wonder whether it wouldn’t be just easier to have two separate commands.

Max

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