On Tue, 22 Sep 2020 13:12:59 +0200 "Roger Pau Monné" <roger.pau@xxxxxxxxxx> wrote:
On Tue, Sep 22, 2020 at 12:52:07PM +0200, 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 | 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;
+
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.