Re: [PATCH] svcrdma: Do not send XDR roundup bytes for a write chunk

From: "J. Bruce Fields" <bfields@fieldses.org>
To: Chuck Lever <chuck.lever@oracle.com>
Cc: linux-nfs@vger.kernel.org, linux-rmda@vger.kernel.org
Subject: Re: [PATCH] svcrdma: Do not send XDR roundup bytes for a write chunk
Date: Thu, 12 Nov 2015 14:13:03 -0500	[thread overview]
Message-ID: <20151112191303.GB8538@fieldses.org> (raw)
In-Reply-To: <20151112144433.13507.34603.stgit@klimt.1015granger.net>

Thanks, applying.

On Thu, Nov 12, 2015 at 09:44:33AM -0500, Chuck Lever wrote:
> Minor optimization:

Based on that I'm queueing it up for 4.5--let me know if that causes
some problem.

--b.

> 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>
> ---
>  net/sunrpc/xprtrdma/svc_rdma_sendto.c |    7 +++++++
>  1 file changed, 7 insertions(+)
> 
> 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,