iser-target: Avoid using FRMR for single dma entry requests

This patch changes isert_reg_rdma_frwr() to not use FRMR for single
dma entry requests from small I/Os, in order to avoid the associated
memory registration overhead.

Using DMA MR is sufficient here for the single dma entry requests,
and addresses a >= v3.12 performance regression.

Signed-off-by: Vu Pham <vu@mellanox.com>
Cc: <stable@vger.kernel.org> # v3.12+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

1 files changed, 20 insertions, 12 deletions

diff --git a/drivers/infiniband/ulp/isert/ib_isert.c b/drivers/infiniband/ulp/isert/ib_isert.c
index 27708c3430d5..bbd86e82e042 100644
--- a/drivers/infiniband/ulp/isert/ib_isert.c
+++ b/drivers/infiniband/ulp/isert/ib_isert.c
@@ -2297,18 +2297,26 @@ isert_reg_rdma_frwr(struct iscsi_conn *conn, struct iscsi_cmd *cmd,
         data_len = min(data_left, rdma_write_max);
         wr->cur_rdma_length = data_len;
 
-        spin_lock_irqsave(&isert_conn->conn_lock, flags);
-        fr_desc = list_first_entry(&isert_conn->conn_frwr_pool,
-                                   struct fast_reg_descriptor, list);
-        list_del(&fr_desc->list);
-        spin_unlock_irqrestore(&isert_conn->conn_lock, flags);
-        wr->fr_desc = fr_desc;
-
-        ret = isert_fast_reg_mr(fr_desc, isert_cmd, isert_conn,
-                          ib_sge, offset, data_len);
-        if (ret) {
-                list_add_tail(&fr_desc->list, &isert_conn->conn_frwr_pool);
-                goto unmap_sg;
+        /* if there is a single dma entry, dma mr is sufficient */
+        if (count == 1) {
+                ib_sge->addr = ib_sg_dma_address(ib_dev, &sg_start[0]);
+                ib_sge->length = ib_sg_dma_len(ib_dev, &sg_start[0]);
+                ib_sge->lkey = isert_conn->conn_mr->lkey;
+                wr->fr_desc = NULL;
+        } else {
+                spin_lock_irqsave(&isert_conn->conn_lock, flags);
+                fr_desc = list_first_entry(&isert_conn->conn_frwr_pool,
+                                           struct fast_reg_descriptor, list);
+                list_del(&fr_desc->list);
+                spin_unlock_irqrestore(&isert_conn->conn_lock, flags);
+                wr->fr_desc = fr_desc;
+
+                ret = isert_fast_reg_mr(fr_desc, isert_cmd, isert_conn,
+                                  ib_sge, offset, data_len);
+                if (ret) {
+                        list_add_tail(&fr_desc->list, &isert_conn->conn_frwr_pool);
+                        goto unmap_sg;
+                }
         }
 
         return 0;