On Tue, 22 Sep 2020 14:11:32 +0200 "Jürgen Groß" [off-list ref] wrote:

On 22.09.20 12:52, SeongJae Park wrote:

quoted

From: SeongJae Park <redacted>

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.  It can be disabled from blkback side using a
module parameter, 'feature_persistent'.  But, it is impossible from
blkfront side.  For the reason, this commit adds a blkfront module
parameter for disabling of the feature.

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

Signed-off-by: SeongJae Park <redacted>
---
  .../ABI/testing/sysfs-driver-xen-blkfront     |  9 ++++++
  drivers/block/xen-blkfront.c                  | 28 +++++++++++++------
  2 files changed, 29 insertions(+), 8 deletions(-)

[...]

quoted

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 91de2e0755ae..49c324f377de 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c

@@ -149,6 +149,13 @@ static unsigned int xen_blkif_max_ring_order;
  module_param_named(max_ring_page_order, xen_blkif_max_ring_order, int, 0444);
  MODULE_PARM_DESC(max_ring_page_order, "Maximum order of pages to be used for the shared ring");
  
+/* 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");
+
+
  #define BLK_RING_SIZE(info)	\
  	__CONST_RING_SIZE(blkif, XEN_PAGE_SIZE * (info)->nr_ring_pages)
  

@@ -1866,11 +1873,13 @@ static int talk_to_blkback(struct xenbus_device *dev,
  		message = "writing protocol";
  		goto abort_transaction;
  	}
-	err = xenbus_printf(xbt, dev->nodename,
-			    "feature-persistent", "%u", 1);
-	if (err)
-		dev_warn(&dev->dev,
-			 "writing persistent grants feature to xenbus");
+	if (feature_persistent) {
+		err = xenbus_printf(xbt, dev->nodename,
+				    "feature-persistent", "%u", 1);
+		if (err)
+			dev_warn(&dev->dev,
+				 "writing persistent grants feature to xenbus");
+	}
  
  	err = xenbus_transaction_end(xbt, 0);
  	if (err) {

@@ -2316,9 +2325,12 @@ static void blkfront_gather_backend_features(struct blkfront_info *info)
  	if (xenbus_read_unsigned(info->xbdev->otherend, "feature-discard", 0))
  		blkfront_setup_discard(info);
  
-	info->feature_persistent =
-		!!xenbus_read_unsigned(info->xbdev->otherend,
-				       "feature-persistent", 0);
+	if (feature_persistent)
+		info->feature_persistent =
+			!!xenbus_read_unsigned(info->xbdev->otherend,
+					       "feature-persistent", 0);
+	else
+		info->feature_persistent = 0;
  
  	indirect_segments = xenbus_read_unsigned(info->xbdev->otherend,
  					"feature-max-indirect-segments", 0);

Here you have the same problem as in blkback: feature_persistent could
change its value between the two tests.

Yes, indeed.  I will fix this in the next version.


Thanks,
SeongJae Park