Re: [PATCH v2 05/22] SUNRPC: Separate buffer pointers for RPC Call and Reply messages

[Anna Schumaker <Anna.Schumaker-HgOvQuBEEgTQT0dZR+AlfA@public.gmane.org>]

Hi Chuck,

On 08/23/2016 01:52 PM, Chuck Lever wrote:
> For xprtrdma, the RPC Call and Reply buffers are involved in real
> I/O operations.
> 
> To start with, the DMA direction of the I/O for a Call is opposite
> that of a Reply.
> 
> In the current arrangement, the Reply buffer address is on a
> four-byte alignment just past the call buffer. Would be friendlier
> on some platforms if that was at a DMA cache alignment instead.
> 
> Because the current arrangement allocates a single memory region
> which contains both buffers, the RPC Reply buffer often contains a
> page boundary in it when the Call buffer is large enough (which is
> frequent).
> 
> It would be a little nicer for setting up DMA operations (and
> possible registration of the Reply buffer) if the two buffers were
> separated, well-aligned, and contained as few page boundaries as
> possible.
> 
> Now, I could just pad out the single memory region used for the pair
> of buffers. But frequently that would mean a lot of unused space to
> ensure the Reply buffer did not have a page boundary.
> 
> Add a separate pointer to rpc_rqst that points right to the RPC
> Reply buffer. This makes no difference to xprtsock, but it will help
> xprtrdma in subsequent patches.
> 
> Signed-off-by: Chuck Lever <chuck.lever-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>
> ---
>  include/linux/sunrpc/xprt.h     |    5 +++--
>  net/sunrpc/clnt.c               |    2 +-
>  net/sunrpc/sched.c              |    1 +
>  net/sunrpc/xprtrdma/transport.c |    1 +
>  4 files changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/sunrpc/xprt.h b/include/linux/sunrpc/xprt.h
> index 72c2aeb..46f069e 100644
> --- a/include/linux/sunrpc/xprt.h
> +++ b/include/linux/sunrpc/xprt.h
> @@ -84,8 +84,9 @@ struct rpc_rqst {
>  	struct list_head	rq_list;
>  
>  	void			*rq_buffer;	/* Call XDR encode buffer */
> -	size_t			rq_callsize,
> -				rq_rcvsize;
> +	size_t			rq_callsize;
> +	void			*rq_rbuffer;	/* Reply XDR decode buffer */
> +	size_t			rq_rcvsize;

Just a nit-picky question :)  Is there any reason that you're adding the buffer between rq_callsize and rq_rcvsize?  It seems like you could leave those alone and add the pointer either before or after them.

Thanks,
Anna

>  	size_t			rq_xmit_bytes_sent;	/* total bytes sent */
>  	size_t			rq_reply_bytes_recvd;	/* total reply bytes */
>  							/* received */
> diff --git a/net/sunrpc/clnt.c b/net/sunrpc/clnt.c
> index ab467c0..fd389c0 100644
> --- a/net/sunrpc/clnt.c
> +++ b/net/sunrpc/clnt.c
> @@ -1768,7 +1768,7 @@ rpc_xdr_encode(struct rpc_task *task)
>  		     req->rq_buffer,
>  		     req->rq_callsize);
>  	xdr_buf_init(&req->rq_rcv_buf,
> -		     (char *)req->rq_buffer + req->rq_callsize,
> +		     req->rq_rbuffer,
>  		     req->rq_rcvsize);
>  
>  	p = rpc_encode_header(task);
> diff --git a/net/sunrpc/sched.c b/net/sunrpc/sched.c
> index 6690ebc..5db68b3 100644
> --- a/net/sunrpc/sched.c
> +++ b/net/sunrpc/sched.c
> @@ -891,6 +891,7 @@ int rpc_malloc(struct rpc_task *task)
>  	dprintk("RPC: %5u allocated buffer of size %zu at %p\n",
>  			task->tk_pid, size, buf);
>  	rqst->rq_buffer = buf->data;
> +	rqst->rq_rbuffer = (char *)rqst->rq_buffer + rqst->rq_callsize;
>  	return 0;
>  }
>  EXPORT_SYMBOL_GPL(rpc_malloc);
> diff --git a/net/sunrpc/xprtrdma/transport.c b/net/sunrpc/xprtrdma/transport.c
> index ebf14ba..136caf3 100644
> --- a/net/sunrpc/xprtrdma/transport.c
> +++ b/net/sunrpc/xprtrdma/transport.c
> @@ -524,6 +524,7 @@ out:
>  	dprintk("RPC:       %s: size %zd, request 0x%p\n", __func__, size, req);
>  	req->rl_connect_cookie = 0;	/* our reserved value */
>  	rqst->rq_buffer = req->rl_sendbuf->rg_base;
> +	rqst->rq_rbuffer = (char *)rqst->rq_buffer + rqst->rq_rcvsize;
>  	return 0;
>  
>  out_rdmabuf:
> 
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