Re: [PATCH v3 1/3] xen-blkback: add a parameter for disabling of persistent grants

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On 22.09.20 16:15, SeongJae Park wrote:
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            | 22 ++++++++++++++-----
  2 files changed, 25 insertions(+), 6 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..f4c8827fa0ad 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -474,6 +474,12 @@ static void xen_vbd_free(struct xen_vbd *vbd)
  	vbd->bdev = NULL;
  }
+/* 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");
+
  static int xen_vbd_create(struct xen_blkif *blkif, blkif_vdev_t handle,
  			  unsigned major, unsigned minor, int readonly,
  			  int cdrom)
@@ -519,6 +525,8 @@ static int xen_vbd_create(struct xen_blkif *blkif, blkif_vdev_t handle,
  	if (q && blk_queue_secure_erase(q))
  		vbd->discard_secure = true;
+ vbd->feature_gnt_persistent = feature_persistent ? 1 : 0;

Just assign the value instead of using the ternary operator?

With that changed you can add my

Reviewed-by: Juergen Gross <jgross@xxxxxxxx>


Juergen



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