Re: [PATCH] svcrdma: Advertise the correct max payload

["J. Bruce Fields" <bfields@fieldses.org>]

On Mon, Sep 22, 2014 at 01:42:07PM -0500, Steve Wise wrote:
> On 9/22/2014 1:39 PM, J. Bruce Fields wrote:
> >On Mon, Sep 22, 2014 at 01:36:53PM -0500, Steve Wise wrote:
> >>Svcrdma currently advertises 1MB, which is too large.  The correct value
> >>is the max scatter-gather allowed in an NFSRDMA IO chunk * the host page
> >>size. This bug is usually benign because the Linux X64 NFSRDMA client
> >>correctly limits the payload size to the correct value (64*4096 = 256KB).
> >>But if the Linux client is PPC64 with a 64KB page size, then the client
> >>will indeed use a payload size that will overflow the server.
> >>
> >>Signed-off-by: Steve Wise <swise@opengridcomputing.com>
> >>---
> >>
> >>  net/sunrpc/xprtrdma/svc_rdma_transport.c |    2 +-
> >>  net/sunrpc/xprtrdma/xprt_rdma.h          |    2 ++
> >>  2 files changed, 3 insertions(+), 1 deletions(-)
> >>
> >>diff --git a/net/sunrpc/xprtrdma/svc_rdma_transport.c b/net/sunrpc/xprtrdma/svc_rdma_transport.c
> >>index 374feb4..4e61880 100644
> >>--- a/net/sunrpc/xprtrdma/svc_rdma_transport.c
> >>+++ b/net/sunrpc/xprtrdma/svc_rdma_transport.c
> >>@@ -91,7 +91,7 @@ struct svc_xprt_class svc_rdma_class = {
> >>  	.xcl_name = "rdma",
> >>  	.xcl_owner = THIS_MODULE,
> >>  	.xcl_ops = &svc_rdma_ops,
> >>-	.xcl_max_payload = RPCSVC_MAXPAYLOAD_TCP,
> >>+	.xcl_max_payload = RPCSVC_MAXPAYLOAD_RDMA,
> >>  	.xcl_ident = XPRT_TRANSPORT_RDMA,
> >>  };
> >>diff --git a/net/sunrpc/xprtrdma/xprt_rdma.h b/net/sunrpc/xprtrdma/xprt_rdma.h
> >>index c419498..467a77c 100644
> >>--- a/net/sunrpc/xprtrdma/xprt_rdma.h
> >>+++ b/net/sunrpc/xprtrdma/xprt_rdma.h
> >>@@ -392,4 +392,6 @@ extern struct kmem_cache *svc_rdma_ctxt_cachep;
> >>  /* Workqueue created in svc_rdma.c */
> >>  extern struct workqueue_struct *svc_rdma_wq;
> >>+#define RPCSVC_MAXPAYLOAD_RDMA	(RPCRDMA_MAX_DATA_SEGS << PAGE_SHIFT)
> >Do you want to define this as the minimum of this and
> >RPCSVC_MAXPAYLOAD_TCP, in case RPCRDMA_MAX_DATA_SEGS gets increased some
> >day?
> 
> Why would it need to be limited by MAXPAYLOAD_TCP?

Because you're also limited by the size of the rq_pages array, which is
determined by RPCSVC_MAXPAGES, calculated from RPCSVC_MAXPAYLOAD.

(Actually you probably want RPCSVC_MAXPAYLOAD, not MAXPAYLOAD_TCP.)

--b.