IB/iser: Do not reduce max_sectors

The iSER driver reduces max_sectors. For example, if you load the
ib_iser module with max_sectors=1024, you will see that
/sys/class/block/<bdev>/queue/max_hw_sectors_kb is 508. It is an
incorrect value. The expected value is (max_sectors * sector_size) /
1024 = 512.

Reducing of max_sectors can cause performance degradation due to
unnecessary splitting of IO requests.

The number of pages per MR has been fixed here, so there is no longer
any need to reduce max_sectors.

Fixes: 9c674815 ("IB/iser: Fix max_sectors calculation")
Signed-off-by: Sergey Gorenko <sergeygo@mellanox.com>
Reviewed-by: Israel Rukshin <israelr@mellanox.com>
Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Sagi Grimberg <sagi@grimberg.me>
Acked-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Doug Ledford <dledford@redhat.com>

drivers/infiniband/ulp/iser/iscsi_iser.c

@@ -665,19 +665,17 @@ iscsi_iser_session_create(struct iscsi_endpoint *ep,
 			goto free_host;
 	}
 
-	/*
-	 * FRs or FMRs can only map up to a (device) page per entry, but if the
-	 * first entry is misaligned we'll end up using using two entries
-	 * (head and tail) for a single page worth data, so we have to drop
-	 * one segment from the calculation.
-	 */
-	max_fr_sectors = ((shost->sg_tablesize - 1) * PAGE_SIZE) >> 9;
+	max_fr_sectors = (shost->sg_tablesize * PAGE_SIZE) >> 9;
 	shost->max_sectors = min(iser_max_sectors, max_fr_sectors);
 
 	iser_dbg("iser_conn %p, sg_tablesize %u, max_sectors %u\n",
 		 iser_conn, shost->sg_tablesize,
 		 shost->max_sectors);
 
+	if (shost->max_sectors < iser_max_sectors)
+		iser_warn("max_sectors was reduced from %u to %u\n",
+			  iser_max_sectors, shost->max_sectors);
+
 	if (cmds_max > max_cmds) {
 		iser_info("cmds_max changed from %u to %u\n",
 			  cmds_max, max_cmds);

drivers/infiniband/ulp/iser/iscsi_iser.h

@@ -498,6 +498,7 @@ struct ib_conn {
  * @rx_descs:         rx buffers array (cyclic buffer)
  * @num_rx_descs:     number of rx descriptors
  * @scsi_sg_tablesize: scsi host sg_tablesize
+ * @pages_per_mr:     maximum pages available for registration
  */
 struct iser_conn {
 	struct ib_conn		     ib_conn;
@@ -520,6 +521,7 @@ struct iser_conn {
 	struct iser_rx_desc	     *rx_descs;
 	u32                          num_rx_descs;
 	unsigned short               scsi_sg_tablesize;
+	unsigned short               pages_per_mr;
 	bool			     snd_w_inv;
 };
 

drivers/infiniband/ulp/iser/iser_initiator.c

@@ -251,7 +251,7 @@ int iser_alloc_rx_descriptors(struct iser_conn *iser_conn,
 	iser_conn->min_posted_rx = iser_conn->qp_max_recv_dtos >> 2;
 
 	if (device->reg_ops->alloc_reg_res(ib_conn, session->scsi_cmds_max,
-					   iser_conn->scsi_sg_tablesize))
+					   iser_conn->pages_per_mr))
 		goto create_rdma_reg_res_failed;
 
 	if (iser_alloc_login_buf(iser_conn))

drivers/infiniband/ulp/iser/iser_verbs.c

@@ -703,19 +703,34 @@ iser_calc_scsi_params(struct iser_conn *iser_conn,
 		      unsigned int max_sectors)
 {
 	struct iser_device *device = iser_conn->ib_conn.device;
+	struct ib_device_attr *attr = &device->ib_device->attrs;
 	unsigned short sg_tablesize, sup_sg_tablesize;
+	unsigned short reserved_mr_pages;
+
+	/*
+	 * FRs without SG_GAPS or FMRs can only map up to a (device) page per
+	 * entry, but if the first entry is misaligned we'll end up using two
+	 * entries (head and tail) for a single page worth data, so one
+	 * additional entry is required.
+	 */
+	if ((attr->device_cap_flags & IB_DEVICE_MEM_MGT_EXTENSIONS) &&
+	    (attr->device_cap_flags & IB_DEVICE_SG_GAPS_REG))
+		reserved_mr_pages = 0;
+	else
+		reserved_mr_pages = 1;
 
 	sg_tablesize = DIV_ROUND_UP(max_sectors * 512, SIZE_4K);
-	if (device->ib_device->attrs.device_cap_flags &
-			IB_DEVICE_MEM_MGT_EXTENSIONS)
+	if (attr->device_cap_flags & IB_DEVICE_MEM_MGT_EXTENSIONS)
 		sup_sg_tablesize =
 			min_t(
 			 uint, ISCSI_ISER_MAX_SG_TABLESIZE,
-			 device->ib_device->attrs.max_fast_reg_page_list_len);
+			 attr->max_fast_reg_page_list_len - reserved_mr_pages);
 	else
 		sup_sg_tablesize = ISCSI_ISER_MAX_SG_TABLESIZE;
 
 	iser_conn->scsi_sg_tablesize = min(sg_tablesize, sup_sg_tablesize);
+	iser_conn->pages_per_mr =
+		iser_conn->scsi_sg_tablesize + reserved_mr_pages;
 }
 
 /**