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Subject: Re: [PATCH] svcrdma: Do not send XDR roundup bytes for a write chunk
From: Chuck Lever <chuck.lever@oracle.com>
In-Reply-To: <20151016132938.8597.65237.stgit@oracle-120.nfsv4bat.org>
Date: Tue, 3 Nov 2015 15:10:21 -0500
Message-Id: <94E65795-E205-44FE-93DE-A1FF24CD5C2E@oracle.com>
References: <20151016132938.8597.65237.stgit@oracle-120.nfsv4bat.org>
To: Linux NFS Mailing List <linux-nfs@vger.kernel.org>,
        linux-rdma@vger.kernel.org
Sender: linux-nfs-owner@vger.kernel.org


> On Oct 16, 2015, at 9:30 AM, Chuck Lever <chuck.lever@oracle.com> wrote:
> 
> Minor optimization: when dealing with write chunk XDR roundup, do
> not post a Write WR for the zero bytes in the pad. Simply update
> the write segment in the RPC-over-RDMA header to reflect the extra
> pad bytes.
> 
> The Reply chunk is also a write chunk, but the server does not use
> send_write_chunks() to send the Reply chunk. That's OK in this case:
> the server Upper Layer typically marshals the Reply chunk contents
> in a single contiguous buffer, without a separate tail for the XDR
> pad.
> 
> The comments and the variable naming refer to "chunks" but what is
> really meant is "segments." The existing code sends only one
> xdr_write_chunk per RPC reply.
> 
> The fix assumes this as well. When the XDR pad in the first write
> chunk is reached, the assumption is the Write list is complete and
> send_write_chunks() returns.
> 
> That will remain a valid assumption until the server Upper Layer can
> support multiple bulk payload results per RPC.
> 
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> --

Bruce, can you take this for 4.4 ?


> net/sunrpc/xprtrdma/svc_rdma_sendto.c |    7 +++++++
> 1 files changed, 7 insertions(+), 0 deletions(-)
> 
> diff --git a/net/sunrpc/xprtrdma/svc_rdma_sendto.c b/net/sunrpc/xprtrdma/svc_rdma_sendto.c
> index 1dfae83..6c48901 100644
> --- a/net/sunrpc/xprtrdma/svc_rdma_sendto.c
> +++ b/net/sunrpc/xprtrdma/svc_rdma_sendto.c
> @@ -342,6 +342,13 @@ static int send_write_chunks(struct svcxprt_rdma *xprt,
> 						arg_ch->rs_handle,
> 						arg_ch->rs_offset,
> 						write_len);
> +
> +		/* Do not send XDR pad bytes */
> +		if (chunk_no && write_len < 4) {
> +			chunk_no++;
> +			break;
> +		}
> +
> 		chunk_off = 0;
> 		while (write_len) {
> 			ret = send_write(xprt, rqstp,
> 
> --
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—
Chuck Lever