Re: [PATCH v1 05/10] svcrdma: Find rmsgp more reliably

[Sagi Grimberg <sagig@dev.mellanox.co.il>]

On 1/9/2015 9:22 PM, Chuck Lever wrote:
> xdr_start() can return the wrong rmsgp address if an assumption
> about how the xdr_buf was constructed changes.  When it gets it
> wrong, the client receives a reply that has gibberish in the
> RPC/RDMA header, preventing it from matching a waiting RPC request.
>
> Instead, make (and document) just one assumption: that the RDMA
> header for the client's RPC call is at the start of the first page
> in rq_pages.

Would it make more sense to add another pointer assigned at req
initialization (maybe in the RDMA request context) instead of hard
coding this assumption? I may be completely wrong here though...

>
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> ---
>
>   net/sunrpc/xprtrdma/svc_rdma_sendto.c |   18 ++++--------------
>   1 files changed, 4 insertions(+), 14 deletions(-)
>
> diff --git a/net/sunrpc/xprtrdma/svc_rdma_sendto.c b/net/sunrpc/xprtrdma/svc_rdma_sendto.c
> index 7d79897..7de33d1 100644
> --- a/net/sunrpc/xprtrdma/svc_rdma_sendto.c
> +++ b/net/sunrpc/xprtrdma/svc_rdma_sendto.c
> @@ -483,18 +483,6 @@ void svc_rdma_prep_reply_hdr(struct svc_rqst *rqstp)
>   {
>   }
>
> -/*
> - * Return the start of an xdr buffer.
> - */
> -static void *xdr_start(struct xdr_buf *xdr)
> -{
> -	return xdr->head[0].iov_base -
> -		(xdr->len -
> -		 xdr->page_len -
> -		 xdr->tail[0].iov_len -
> -		 xdr->head[0].iov_len);
> -}
> -
>   int svc_rdma_sendto(struct svc_rqst *rqstp)
>   {
>   	struct svc_xprt *xprt = rqstp->rq_xprt;
> @@ -512,8 +500,10 @@ int svc_rdma_sendto(struct svc_rqst *rqstp)
>
>   	dprintk("svcrdma: sending response for rqstp=%p\n", rqstp);
>
> -	/* Get the RDMA request header. */
> -	rdma_argp = xdr_start(&rqstp->rq_arg);
> +	/* Get the RDMA request header. The receive logic always
> +	 * places this at the start of page 0.
> +	 */
> +	rdma_argp = page_address(rqstp->rq_pages[0]);
>
>   	/* Build an req vec for the XDR */
>   	ctxt = svc_rdma_get_context(rdma);
>
> --
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