[PATCH v2 1/3] xen-blkback: add a parameter for disabling of persistent grants

[SeongJae Park <sjpark@xxxxxxxxxx>]

From: SeongJae Park <sjpark@xxxxxxxxx>

Persistent grants feature provides high scalability.  On some small
systems, however, it could incur data copy overheads[1] and thus it is
required to be disabled.  But, there is no option to disable it.  For
the reason, this commit adds a module parameter for disabling of the
feature.

[1] https://wiki.xen.org/wiki/Xen_4.3_Block_Protocol_Scalability

Signed-off-by: Anthony Liguori <aliguori@xxxxxxxxxx>
Signed-off-by: SeongJae Park <sjpark@xxxxxxxxx>
---
 .../ABI/testing/sysfs-driver-xen-blkback      |  9 ++++++
 drivers/block/xen-blkback/xenbus.c            | 28 ++++++++++++++-----
 2 files changed, 30 insertions(+), 7 deletions(-)

diff --git a/Documentation/ABI/testing/sysfs-driver-xen-blkback 
b/Documentation/ABI/testing/sysfs-driver-xen-blkback
index ecb7942ff146..ac2947b98950 100644
--- a/Documentation/ABI/testing/sysfs-driver-xen-blkback
+++ b/Documentation/ABI/testing/sysfs-driver-xen-blkback
@@ -35,3 +35,12 @@ Description:
                 controls the duration in milliseconds that blkback will not
                 cache any page not backed by a grant mapping.
                 The default is 10ms.
+
+What:           /sys/module/xen_blkback/parameters/feature_persistent
+Date:           September 2020
+KernelVersion:  5.10
+Contact:        SeongJae Park <sjpark@xxxxxxxxx>
+Description:
+                Whether to enable the persistent grants feature or not.  Note
+                that this option only takes effect on newly created backends.
+                The default is Y (enable).
diff --git a/drivers/block/xen-blkback/xenbus.c 
b/drivers/block/xen-blkback/xenbus.c
index b9aa5d1ac10b..8a95ddd08b13 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -879,6 +879,12 @@ static void reclaim_memory(struct xenbus_device *dev)
 
 /* ** Connection ** */
 
+/* Enable the persistent grants feature. */
+static bool feature_persistent = true;
+module_param(feature_persistent, bool, 0644);
+MODULE_PARM_DESC(feature_persistent,
+               "Enables the persistent grants feature");
+
 /*
  * Write the physical details regarding the block device to the store, and
  * switch to Connected state.
@@ -906,11 +912,15 @@ static void connect(struct backend_info *be)
 
        xen_blkbk_barrier(xbt, be, be->blkif->vbd.flush_support);
 
-       err = xenbus_printf(xbt, dev->nodename, "feature-persistent", "%u", 1);
-       if (err) {
-               xenbus_dev_fatal(dev, err, "writing %s/feature-persistent",
-                                dev->nodename);
-               goto abort;
+       if (feature_persistent) {
+               err = xenbus_printf(xbt, dev->nodename, "feature-persistent",
+                               "%u", feature_persistent);
+               if (err) {
+                       xenbus_dev_fatal(dev, err,
+                                       "writing %s/feature-persistent",
+                                       dev->nodename);
+                       goto abort;
+               }
        }
 
        err = xenbus_printf(xbt, dev->nodename, "sectors", "%llu",
@@ -1093,8 +1103,12 @@ static int connect_ring(struct backend_info *be)
                xenbus_dev_fatal(dev, err, "unknown fe protocol %s", protocol);
                return -ENOSYS;
        }
-       pers_grants = xenbus_read_unsigned(dev->otherend, "feature-persistent",
-                                          0);
+       if (feature_persistent)
+               pers_grants = xenbus_read_unsigned(dev->otherend,
+                               "feature-persistent", 0);
+       else
+               pers_grants = 0;
+
        blkif->vbd.feature_gnt_persistent = pers_grants;
        blkif->vbd.overflow_max_grants = 0;
 
-- 
2.17.1