svcrdma: Reduce size of struct svc_rdma_rw_ctxt

SG_CHUNK_SIZE is 128, making struct svc_rdma_rw_ctxt + the first
SGL array more than 4200 bytes in length, pushing the memory
allocation well into order 1.

Even so, the RDMA rw core doesn't seem to use more than max_send_sge
entries in that array (typically 32 or less), so that is all wasted
space.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

net/sunrpc/xprtrdma/svc_rdma_rw.c

@@ -39,6 +39,7 @@ struct svc_rdma_rw_ctxt {
 	struct list_head	rw_list;
 	struct rdma_rw_ctx	rw_ctx;
 	unsigned int		rw_nents;
+	unsigned int		rw_first_sgl_nents;
 	struct sg_table		rw_sg_table;
 	struct scatterlist	rw_first_sgl[];
 };
@@ -53,6 +54,8 @@ svc_rdma_next_ctxt(struct list_head *list)
 static struct svc_rdma_rw_ctxt *
 svc_rdma_get_rw_ctxt(struct svcxprt_rdma *rdma, unsigned int sges)
 {
+	struct ib_device *dev = rdma->sc_cm_id->device;
+	unsigned int first_sgl_nents = dev->attrs.max_send_sge;
 	struct svc_rdma_rw_ctxt *ctxt;
 	struct llist_node *node;
 
@@ -62,18 +65,19 @@ svc_rdma_get_rw_ctxt(struct svcxprt_rdma *rdma, unsigned int sges)
 	if (node) {
 		ctxt = llist_entry(node, struct svc_rdma_rw_ctxt, rw_node);
 	} else {
-		ctxt = kmalloc_node(struct_size(ctxt, rw_first_sgl, SG_CHUNK_SIZE),
-				    GFP_KERNEL, ibdev_to_node(rdma->sc_cm_id->device));
+		ctxt = kmalloc_node(struct_size(ctxt, rw_first_sgl, first_sgl_nents),
+				    GFP_KERNEL, ibdev_to_node(dev));
 		if (!ctxt)
 			goto out_noctx;
 
 		INIT_LIST_HEAD(&ctxt->rw_list);
+		ctxt->rw_first_sgl_nents = first_sgl_nents;
 	}
 
 	ctxt->rw_sg_table.sgl = ctxt->rw_first_sgl;
 	if (sg_alloc_table_chained(&ctxt->rw_sg_table, sges,
 				   ctxt->rw_sg_table.sgl,
-				   SG_CHUNK_SIZE))
+				   first_sgl_nents))
 		goto out_free;
 	return ctxt;
 
@@ -87,7 +91,7 @@ svc_rdma_get_rw_ctxt(struct svcxprt_rdma *rdma, unsigned int sges)
 static void __svc_rdma_put_rw_ctxt(struct svc_rdma_rw_ctxt *ctxt,
 				   struct llist_head *list)
 {
-	sg_free_table_chained(&ctxt->rw_sg_table, SG_CHUNK_SIZE);
+	sg_free_table_chained(&ctxt->rw_sg_table, ctxt->rw_first_sgl_nents);
 	llist_add(&ctxt->rw_node, list);
 }