Patch "svcrdma: Fix send_reply() scatter/gather set-up" has been added to the 4.1-stable tree

This is a note to let you know that I've just added the patch titled

svcrdma: Fix send_reply() scatter/gather set-up

to the 4.1-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
svcrdma-fix-send_reply-scatter-gather-set-up.patch
and it can be found in the queue-4.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Chuck Lever <[email protected]>
Date: Thu, 9 Jul 2015 16:45:18 -0400
Subject: svcrdma: Fix send_reply() scatter/gather set-up

From: Chuck Lever <[email protected]>

commit 9d11b51ce7c150a69e761e30518f294fc73d55ff upstream.

The Linux NFS server returns garbage in the data payload of inline NFS/RDMA READ replies. These are READs of under 1000 bytes or so where the client has not provided either a reply chunk or a write list.

The NFS server delivers the data payload for an NFS READ reply to the transport in an xdr_buf page list. If the NFS client did not provide a reply chunk or a write list, send_reply() is supposed to set up a separate sge for the page containing the READ data, and another sge for XDR padding if needed, then post all of the sges via a single SEND Work Request.

The problem is send_reply() does not advance through the xdr_buf when setting up scatter/gather entries for SEND WR. It always calls dma_map_xdr with xdr_off set to zero. When there's more than one sge, dma_map_xdr() sets up the SEND sge's so they all point to the xdr_buf's head.

The current Linux NFS/RDMA client always provides a reply chunk or a write list when performing an NFS READ over RDMA. Therefore, it does not exercise this particular case. The Linux server has never had to use more than one extra sge for building RPC/RDMA replies with a Linux client.

However, an NFS/RDMA client _is_ allowed to send small NFS READs without setting up a write list or reply chunk. The NFS READ reply fits entirely within the inline reply buffer in this case. This is perhaps a more efficient way of performing NFS READs that the Linux NFS/RDMA client may some day adopt.

---
 net/sunrpc/xprtrdma/svc_rdma_sendto.c |   10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

@@ -382,6 +382,7 @@ static int send_reply(struct svcxprt_rdm
 		      int byte_count)
 {
 	struct ib_send_wr send_wr;
+	u32 xdr_off;
 	int sge_no;
 	int sge_bytes;
 	int page_no;

@@ -416,8 +417,8 @@ static int send_reply(struct svcxprt_rdm
 	ctxt->direction = DMA_TO_DEVICE;
 
 	/* Map the payload indicated by 'byte_count' */
+	xdr_off = 0;
 	for (sge_no = 1; byte_count && sge_no < vec->count; sge_no++) {
-		int xdr_off = 0;
 		sge_bytes = min_t(size_t, vec->sge[sge_no].iov_len, byte_count);
 		byte_count -= sge_bytes;
 		ctxt->sge[sge_no].addr =

@@ -455,6 +456,13 @@ static int send_reply(struct svcxprt_rdm
 	}
 	rqstp->rq_next_page = rqstp->rq_respages + 1;
 
+	/* The loop above bumps sc_dma_used for each sge. The
+	 * xdr_buf.tail gets a separate sge, but resides in the
+	 * same page as xdr_buf.head. Don't count it twice.
+	 */
+	if (sge_no > ctxt->count)
+		atomic_dec(&rdma->sc_dma_used);
+
 	if (sge_no > rdma->sc_max_sge) {
 		pr_err("svcrdma: Too many sges (%d)\n", sge_no);
 		goto err;

Patches currently in stable-queue which might be from [email protected] are

queue-4.1/svcrdma-fix-send_reply-scatter-gather-set-up.patch