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

On Tue, 22 Sep 2020 13:12:59 +0200 "Roger Pau Monné" [off-list ref] wrote:

On Tue, Sep 22, 2020 at 12:52:07PM +0200, 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.  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 <redacted>
Signed-off-by: SeongJae Park <redacted>
---
 .../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@amazon.de>
+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;
+

Sorry for not realizing earlier, but looking at it again I think you
need to cache the value of feature_persistent when it's first used in
the blkback state data, so that it's consistent.

What would happen for example with the following flow (assume a
persistent grants enabled frontend):

feature_persistent = false

connect(...)
feature-persistent is not written to xenstore

User changes feature_persistent = true

connect_ring(...)
pers_grants = true, because feature-persistent is set unconditionally
by the frontend and feature_persistent variable is now true.

Then blkback will try to use persistent grants and the whole
connection will malfunction because the frontend won't.

Ah, you're right.  I should also catch this before but didn't, sorry.

The other option is to prevent changing the variable when there are
blkback instances already running.

I think storing the option value in xenstore would be simpler.  That said, if
you prefer this way, please let me know.


Thanks,
SeongJae Park