[PATCH v4 0/3] xen-blk(back|front): Let users disable persistent grants

[PATCH v4 0/3] xen-blk(back|front): Let users disable persistent grants

[SeongJae Park]

From: SeongJae Park <sjpark@amazon.de>

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 module parameters for disabling of the
feature.

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

Baseline and Complete Git Trees
===============================

The patches are based on the v5.9-rc6.  You can also clone the complete
git tree:

    $ git clone git://github.com/sjp38/linux -b pgrants_disable_v4

The web is also available:
https://github.com/sjp38/linux/tree/pgrants_disable_v4

Patch History
=============

Changes from v3
(https://lore.kernel.org/xen-devel/20200922141549.26154-2-sjpark@amazon.com/)
- Remove unnecessary ternary operation (Jürgen Groß)
- Add 'Reviewed-by: Juergen Gross <jgross@suse.com>' and
  'Acked-by: Roger Pau Monné <roger.pau@citrix.com>'

Changes from v2
(https://lore.kernel.org/linux-block/20200922105209.5284-1-sjpark@amazon.com/)
- Avoid a race condition (Roger Pau Monné)

Because the option value is written twice, there could be a race if the
user change the value meanwhile.  This version fixes the problem by
reading the value once when each (front|back)end is created.

Changes from v1
(https://lore.kernel.org/linux-block/20200922070125.27251-1-sjpark@amazon.com/)
- use 'bool' parameter type (Jürgen Groß)
- Let blkfront can also disable the feature from its side
  (Roger Pau Monné)
- Avoid unnecessary xenbus_printf (Roger Pau Monné)
- Update frontend parameter doc


SeongJae Park (3):
  xen-blkback: add a parameter for disabling of persistent grants
  xen-blkfront: add a parameter for disabling of persistent grants
  xen-blkfront: Apply changed parameter name to the document

 .../ABI/testing/sysfs-driver-xen-blkback      |  9 ++++++++
 .../ABI/testing/sysfs-driver-xen-blkfront     | 11 +++++++++-
 drivers/block/xen-blkback/xenbus.c            | 22 ++++++++++++++-----
 drivers/block/xen-blkfront.c                  | 20 ++++++++++++-----
 4 files changed, 50 insertions(+), 12 deletions(-)

-- 
2.17.1

[PATCH v4 1/3] xen-blkback: add a parameter for disabling of persistent grants

[SeongJae Park]

From: SeongJae Park <sjpark@amazon.de>

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@amazon.com>
Signed-off-by: SeongJae Park <sjpark@amazon.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
---
 .../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@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..8fc34211dc8b 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;
+
 	pr_debug("Successful creation of handle=%04x (dom=%u)\n",
 		handle, blkif->domid);
 	return 0;
@@ -906,7 +914,8 @@ 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);
+	err = xenbus_printf(xbt, dev->nodename, "feature-persistent", "%u",
+			be->blkif->vbd.feature_gnt_persistent);
 	if (err) {
 		xenbus_dev_fatal(dev, err, "writing %s/feature-persistent",
 				 dev->nodename);
@@ -1067,7 +1076,6 @@ static int connect_ring(struct backend_info *be)
 {
 	struct xenbus_device *dev = be->dev;
 	struct xen_blkif *blkif = be->blkif;
-	unsigned int pers_grants;
 	char protocol[64] = "";
 	int err, i;
 	char *xspath;
@@ -1093,9 +1101,11 @@ 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);
-	blkif->vbd.feature_gnt_persistent = pers_grants;
+	if (blkif->vbd.feature_gnt_persistent)
+		blkif->vbd.feature_gnt_persistent =
+			xenbus_read_unsigned(dev->otherend,
+					"feature-persistent", 0);
+
 	blkif->vbd.overflow_max_grants = 0;
 
 	/*
@@ -1118,7 +1128,7 @@ static int connect_ring(struct backend_info *be)
 
 	pr_info("%s: using %d queues, protocol %d (%s) %s\n", dev->nodename,
 		 blkif->nr_rings, blkif->blk_protocol, protocol,
-		 pers_grants ? "persistent grants" : "");
+		 blkif->vbd.feature_gnt_persistent ? "persistent grants" : "");
 
 	ring_page_order = xenbus_read_unsigned(dev->otherend,
 					       "ring-page-order", 0);
-- 
2.17.1

[PATCH v4 2/3] xen-blkfront: add a parameter for disabling of persistent grants

[SeongJae Park]

From: SeongJae Park <sjpark@amazon.de>

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 <sjpark@amazon.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
---
 .../ABI/testing/sysfs-driver-xen-blkfront     |  9 +++++++++
 drivers/block/xen-blkfront.c                  | 20 ++++++++++++++-----
 2 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/Documentation/ABI/testing/sysfs-driver-xen-blkfront b/Documentation/ABI/testing/sysfs-driver-xen-blkfront
index c0a6cb7eb314..9c31334cb2e6 100644
--- a/Documentation/ABI/testing/sysfs-driver-xen-blkfront
+++ b/Documentation/ABI/testing/sysfs-driver-xen-blkfront
@@ -8,3 +8,12 @@ Description:
                 is 32 - higher value means more potential throughput but more
                 memory usage. The backend picks the minimum of the frontend
                 and its default backend value.
+
+What:           /sys/module/xen_blkfront/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 frontends.
+                The default is Y (enable).
diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 91de2e0755ae..48629d3433b4 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -1866,8 +1866,8 @@ 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);
+	err = xenbus_printf(xbt, dev->nodename, "feature-persistent", "%u",
+			info->feature_persistent);
 	if (err)
 		dev_warn(&dev->dev,
 			 "writing persistent grants feature to xenbus");
@@ -1941,6 +1941,13 @@ static int negotiate_mq(struct blkfront_info *info)
 	}
 	return 0;
 }
+
+/* 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");
+
 /**
  * Entry point to this code when a new device is created.  Allocate the basic
  * structures and the ring buffer for communication with the backend, and
@@ -2007,6 +2014,8 @@ static int blkfront_probe(struct xenbus_device *dev,
 	info->vdevice = vdevice;
 	info->connected = BLKIF_STATE_DISCONNECTED;
 
+	info->feature_persistent = feature_persistent;
+
 	/* Front end dir is a number, which is used as the id. */
 	info->handle = simple_strtoul(strrchr(dev->nodename, '/')+1, NULL, 0);
 	dev_set_drvdata(&dev->dev, info);
@@ -2316,9 +2325,10 @@ 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 (info->feature_persistent)
+		info->feature_persistent =
+			!!xenbus_read_unsigned(info->xbdev->otherend,
+					       "feature-persistent", 0);
 
 	indirect_segments = xenbus_read_unsigned(info->xbdev->otherend,
 					"feature-max-indirect-segments", 0);
-- 
2.17.1

[PATCH v4 3/3] xen-blkfront: Apply changed parameter name to the document

[SeongJae Park]

From: SeongJae Park <sjpark@amazon.de>

Commit 14e710fe7897 ("xen-blkfront: rename indirect descriptor
parameter") changed the name of the module parameter for the maximum
amount of segments in indirect requests but missed updating the
document.  This commit updates the document.

Signed-off-by: SeongJae Park <sjpark@amazon.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
---
 Documentation/ABI/testing/sysfs-driver-xen-blkfront | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Documentation/ABI/testing/sysfs-driver-xen-blkfront b/Documentation/ABI/testing/sysfs-driver-xen-blkfront
index 9c31334cb2e6..28008905615f 100644
--- a/Documentation/ABI/testing/sysfs-driver-xen-blkfront
+++ b/Documentation/ABI/testing/sysfs-driver-xen-blkfront
@@ -1,4 +1,4 @@
-What:           /sys/module/xen_blkfront/parameters/max
+What:           /sys/module/xen_blkfront/parameters/max_indirect_segments
 Date:           June 2013
 KernelVersion:  3.11
 Contact:        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
-- 
2.17.1