[PATCH v4 1/9] xprtrdma: Fix Read chunk padding

[Chuck Lever <chuck.lever@oracle.com>]

When pad optimization is disabled, rpcrdma_convert_iovs still
does not add explicit XDR round-up padding to a Read chunk.

Commit 677eb17e94ed ("xprtrdma: Fix XDR tail buffer marshalling")
incorrectly short-circuited the test for whether round-up padding
is needed that appears later in rpcrdma_convert_iovs.

However, if this is indeed a regular Read chunk (and not a
Position-Zero Read chunk), the tail iovec _always_ contains the
chunk's padding, and never anything else.

So, it's easy to just skip the tail when padding optimization is
enabled, and add the tail in a subsequent Read chunk segment, if
disabled.

Fixes: 677eb17e94ed ("xprtrdma: Fix XDR tail buffer marshalling")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 net/sunrpc/xprtrdma/rpc_rdma.c |   10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/net/sunrpc/xprtrdma/rpc_rdma.c b/net/sunrpc/xprtrdma/rpc_rdma.c
index c52e0f2..a524d3c 100644
--- a/net/sunrpc/xprtrdma/rpc_rdma.c
+++ b/net/sunrpc/xprtrdma/rpc_rdma.c
@@ -226,8 +226,10 @@ static bool rpcrdma_results_inline(struct rpcrdma_xprt *r_xprt,
 	if (len && n == RPCRDMA_MAX_SEGS)
 		goto out_overflow;
 
-	/* When encoding the read list, the tail is always sent inline */
-	if (type == rpcrdma_readch)
+	/* When encoding a Read chunk, the tail iovec contains an
+	 * XDR pad and may be omitted.
+	 */
+	if (type == rpcrdma_readch && xprt_rdma_pad_optimize)
 		return n;
 
 	/* When encoding the Write list, some servers need to see an extra
@@ -238,10 +240,6 @@ static bool rpcrdma_results_inline(struct rpcrdma_xprt *r_xprt,
 		return n;
 
 	if (xdrbuf->tail[0].iov_len) {
-		/* the rpcrdma protocol allows us to omit any trailing
-		 * xdr pad bytes, saving the server an RDMA operation. */
-		if (xdrbuf->tail[0].iov_len < 4 && xprt_rdma_pad_optimize)
-			return n;
 		n = rpcrdma_convert_kvec(&xdrbuf->tail[0], seg, n);
 		if (n == RPCRDMA_MAX_SEGS)
 			goto out_overflow;