Re: [PATCH/RFC] NFS: handle NFSv4.1 server that doesn't support NFS4_OPEN_CLAIM_DELEG_CUR_FH

[NeilBrown <neilb@suse.de>]

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On Thu, Dec 19 2019, Trond Myklebust wrote:

> On Thu, 2019-12-19 at 16:39 +1100, NeilBrown wrote:
>> On Thu, Dec 19 2019, Trond Myklebust wrote:
>> 
>> > On Thu, 2019-12-19 at 13:56 +1100, NeilBrown wrote:
>> > > On Wed, Dec 18 2019, Trond Myklebust wrote:
>> > > 
>> > > > On Thu, 2019-12-19 at 09:47 +1100, NeilBrown wrote:
>> > > > > If an NFSv4.1 server doesn't support
>> > > > > NFS4_OPEN_CLAIM_DELEG_CUR_FH
>> > > > > (e.g. Linux 3.0), and a newer NFS client tries to use it to
>> > > > > claim
>> > > > > an open before returning a delegation, the server might
>> > > > > return
>> > > > > NFS4ERR_BADXDR.
>> > > > > That is what Linux 3.0 does, though the RFC doesn't seem to
>> > > > > be
>> > > > > explicit
>> > > > > on which flags must be supported, and what error can be
>> > > > > returned
>> > > > > for
>> > > > > unsupported flags.
>> > > > 
>> > > > NFS4ERR_BADXDR is defined in RFC5661, section 15.1.1.1 as
>> > > > meaning
>> > > > 
>> > > > "The arguments for this operation do not match those specified
>> > > > in
>> > > > the
>> > > > XDR definition."
>> > > > 
>> > > > That's clearly not the case here, so I'd chalk this down to a
>> > > > fairly
>> > > > blatant server bug, at which point it makes no sense to fix it
>> > > > in
>> > > > the
>> > > > client.
>> > > 
>> > > Ok, but the RFC seems to suggest it is OK to not support this
>> > > flag,
>> > > so
>> > > suppose I fixed the server to return NFS4ERR_NOTSUPP instead.
>> > > The client still wouldn't handle this response gracefully.
>> > > 
>> > 
>> > NFS4ERR_NOTSUPP is wrong too as the OPEN operation is clearly
>> > supported. The only error that might make sense is NFS4ERR_INVAL:
>> > 
>> > "15.1.1.4.  NFS4ERR_INVAL (Error Code 22)
>> > 
>> >    The arguments for this operation are not valid for some reason,
>> > even
>> >    though they do match those specified in the XDR definition for
>> > the
>> >    request."
>> > 
>> > That said, why do we care about supporting NFSv4.1 on this server?
>> > It
>> > is clearly broken.
>> 
>> I care about it because a customer has a support contract, but that
>> isn't your problem.
>> 
>> I would think "we" care about it because we want to support the spec,
>> and the spec (RFC 5661 section 2.4) says:
>> 
>>                                                         where the
>> server
>>    supports neither the CLAIM_DELEGATE_PREV nor CLAIM_DELEG_CUR_FH
>> claim
>>    types
>
> Given the context, I think that is actually a typo. It looks to me like
> it is talking about CLAIM_DELEGATE_PREV and CLAIM_DELEG_PREV_FH, since
> otherwise the talk about releasing delegation state when establishing a
> new lease makes no sense.

Hmmm.. Yes, that's believable.

>
>
>> Also you have code in the client to handle the possibility that an
>> NFSv4.1 or later server might not handle some features of OPEN.
>> Three separate features are grouped under "NFS_CAP_ATOMIC_OPEN_V1":
>> If this isn't set, we fall back:
>> 
>>         case NFS4_OPEN_CLAIM_FH:
>>                 return NFS4_OPEN_CLAIM_NULL;
>>         case NFS4_OPEN_CLAIM_DELEG_CUR_FH:
>>                 return NFS4_OPEN_CLAIM_DELEGATE_CUR;
>>         case NFS4_OPEN_CLAIM_DELEG_PREV_FH:
>>                 return NFS4_OPEN_CLAIM_DELEGATE_PREV;
>> 
>
> Right. That's a convenience for downgrading NFSv4.1 service to what is
> supported by NFSv4.0.
>
>> However nfs4_map_atomic_open_claim() is not called when
>> NFS4_OPEN_CLAIM_DELEG_CUR_FH is tried, and fails.  This appears
>> to be an omission in the code.
>> 
>
> It is deliberate. There really isn't anything that describes what is
> and isn't mandatory to implement in NFSv4.1, but if we have to make
> everything optional, then we're going to have to add a lot of mostly
> unnecessary complexity to the client.
> At what point do we then stop? Do we support a NFSv4.1 server that
> implements no NFSv4.1 features? Why not just let the client downgrade
> to NFSv4.0 in that case?

I was a bit surprised that nfs4_map_atomic_open_claim() exists at all,
but given that it did, I assumed it would be used more uniformly.

So this all implies that Linux NFS server claimed to support NFSv4.1
before it actually did - which seems odd.  This is just a bug (which are
expected), but a clear ommission.

Oh well, it probably won't be too hard to backport the
NFS4_OPEN_CLAIM_DELEG_CUR_FH support if it turns out to be really
needed.

Thanks a lot for your time,
NeilBrown


>
>
> -- 
> Trond Myklebust
> Linux NFS client maintainer, Hammerspace
> trond.myklebust@hammerspace.com

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