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From:   Chuck Lever III <chuck.lever@oracle.com>
To:     Trond Myklebust <trondmy@hammerspace.com>
CC:     Olga Kornievskaia <olga.kornievskaia@gmail.com>,
        Linux NFS Mailing List <linux-nfs@vger.kernel.org>,
        Anna Schumaker <anna.schumaker@netapp.com>
Subject: Re: [RFC 1/2] xprtrdma: xdr pad optimization revisted again
Thread-Topic: [RFC 1/2] xprtrdma: xdr pad optimization revisted again
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Date:   Mon, 30 Aug 2021 18:02:26 +0000
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> On Aug 30, 2021, at 1:34 PM, Trond Myklebust <trondmy@hammerspace.com> wr=
ote:
>=20
> On Mon, 2021-08-30 at 13:24 -0400, Olga Kornievskaia wrote:
>> On Mon, Aug 30, 2021 at 1:04 PM Chuck Lever III
>> <chuck.lever@oracle.com> wrote:
>>>=20
>>> Hi Olga-
>>>=20
>>>> On Aug 30, 2021, at 12:53 PM, Olga Kornievskaia
>>>> <olga.kornievskaia@gmail.com> wrote:
>>>>=20
>>>> From: Olga Kornievskaia <kolga@netapp.com>
>>>>=20
>>>> Given the patch "Always provide aligned buffers to the RPC read
>>>> layers",
>>>> RPC over RDMA doesn't need to look at the tail page and add that
>>>> space
>>>> to the write chunk.
>>>>=20
>>>> For the RFC 8166 compliant server, it must not write an XDR
>>>> padding
>>>> into the write chunk (even if space was provided). Historically
>>>> (before RFC 8166) Solaris RDMA server has been requiring the
>>>> client
>>>> to provide space for the XDR padding and thus this client code
>>>> has
>>>> existed.
>>>=20
>>> I don't understand this change.
>>>=20
>>> So, the upper layer doesn't provide XDR padding any more. That
>>> doesn't
>>> mean Solaris servers still aren't going to want to write into it.
>>> The
>>> client still has to provide this padding from somewhere.
>>>=20
>>> This suggests that "Always provide aligned buffers to the RPC read
>>> layers" breaks our interop with Solaris servers. Does it?
>>=20
>> No, I don't believe "Always provide aligned buffers to the RPC read
>> layers" breaks the interoperability. THIS patch would break the
>> interop.
>>=20
>> If we are not willing to break the interoperability and support only
>> servers that comply with RFC 8166, this patch is not needed.
>=20
> Why? The intention of the first patch is to ensure that we do not have
> buffers that are not word aligned. If Solaris wants to write padding
> after the end of the file, then there is space in the page buffer for
> it to do so. There should be no need for an extra tail in which to
> write the padding.

The RPC/RDMA protocol is designed for hardware-offloaded direct data
placement. That means the padding, which isn't data, must be directed
to another buffer.

This is a problem with RPC/RDMA v1 implementations. RFC 5666 was
ambiguous, so there are implementations that write XDR padding into
Write chunks. This is why RFC 8166 says SHOULD NOT instead of MUST
NOT.

I believe rpcrdma-version-two makes it a requirement not to use XDR
padding in either Read or Write data payload chunks.


> This means that the RDMA and TCP cases should end up doing the same
> thing for the case of the Solaris server: the padding is written into
> the page buffer. There is nothing written to the tail in either case.

"Always provide" can guarantee that the NFS client makes aligned
requests for buffered I/O, but what about NFS direct I/O from user
space? The NIC will place the data payload in the application
buffer, but there's no guarantee that the NFS READ request will be
aligned or that the buffer will be able to sink the extra padding
bytes.

We would definitely consider it an error if an unaligned RDMA Read
leaked the link-layer's 4-byte padding into a sink buffer.

So, "Always provide" is nice for the in-kernel NFS client, but I
don't believe it allows the way xprtrdma behaves to be changed.


>>>> Signed-off-by: Olga Kornievskaia <kolga@netapp.com>
>>>> ---
>>>> net/sunrpc/xprtrdma/rpc_rdma.c | 15 ---------------
>>>> 1 file changed, 15 deletions(-)
>>>>=20
>>>> diff --git a/net/sunrpc/xprtrdma/rpc_rdma.c
>>>> b/net/sunrpc/xprtrdma/rpc_rdma.c
>>>> index c335c1361564..2c4146bcf2a8 100644
>>>> --- a/net/sunrpc/xprtrdma/rpc_rdma.c
>>>> +++ b/net/sunrpc/xprtrdma/rpc_rdma.c
>>>> @@ -255,21 +255,6 @@ rpcrdma_convert_iovs(struct rpcrdma_xprt
>>>> *r_xprt, struct xdr_buf *xdrbuf,
>>>>               page_base =3D 0;
>>>>       }
>>>>=20
>>>> -     if (type =3D=3D rpcrdma_readch)
>>>> -             goto out;
>>>> -
>>>> -     /* When encoding a Write chunk, some servers need to see an
>>>> -      * extra segment for non-XDR-aligned Write chunks. The
>>>> upper
>>>> -      * layer provides space in the tail iovec that may be used
>>>> -      * for this purpose.
>>>> -      */
>>>> -     if (type =3D=3D rpcrdma_writech && r_xprt->rx_ep-
>>>>> re_implicit_roundup)
>>>> -             goto out;
>>>> -
>>>> -     if (xdrbuf->tail[0].iov_len)
>>>=20
>>> Instead of checking for a tail, we could check
>>>=20
>>>         if (xdr_pad_size(xdrbuf->page_len))
>>>=20
>>> and provide some tail space in that case.
>>=20
>> I don't believe this is any different than what we have now. If the
>> page size is non-4byte aligned then, we would still allocate size for
>> the padding which "SHOULD NOT" be there. But yes it is allowed to be
>> there.
>>=20
>> The problem, as you know from our offline discussion, is allocating
>> the tail page and including it in the write chunk for the Nvidia
>> environment where Nvidia doesn't support use of data (user) pages and
>> nfs kernel allocated pages in the same segment.
>>=20
>> Alternatively, my ask is then to change rpcrdma_convert_iovs() to
>> return 2 segs instead of one: one for the pages and another for the
>> tail.
>>=20
>>>=20
>>>> -             rpcrdma_convert_kvec(&xdrbuf->tail[0], seg, &n);
>>>> -
>>>> -out:
>>>>       if (unlikely(n > RPCRDMA_MAX_SEGS))
>>>>               return -EIO;
>>>>       return n;
>>>> --
>>>> 2.27.0
>>>>=20
>>>=20
>>> --
>>> Chuck Lever
>>>=20
>>>=20
>>>=20
>=20
> --=20
> Trond Myklebust
> Linux NFS client maintainer, Hammerspace
> trond.myklebust@hammerspace.com

--
Chuck Lever