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    SubjectRe: [PATCH] xen-blkback: add a parameter for disabling of persistent grants
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    On 22.09.20 09:01, SeongJae Park wrote:
    > From: SeongJae Park <sjpark@amazon.de>
    >
    > Persistent grants feature provides high scalability. On some small
    > systems, however, it could incur data copy overhead[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>
    > ---
    > .../ABI/testing/sysfs-driver-xen-blkback | 8 ++++++++
    > drivers/block/xen-blkback/xenbus.c | 17 ++++++++++++++---
    > 2 files changed, 22 insertions(+), 3 deletions(-)
    >
    > diff --git a/Documentation/ABI/testing/sysfs-driver-xen-blkback b/Documentation/ABI/testing/sysfs-driver-xen-blkback
    > index ecb7942ff146..0c42285c75ee 100644
    > --- a/Documentation/ABI/testing/sysfs-driver-xen-blkback
    > +++ b/Documentation/ABI/testing/sysfs-driver-xen-blkback
    > @@ -35,3 +35,11 @@ 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.
    > + The default is 1 (enable).
    > diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
    > index b9aa5d1ac10b..9c03d70469f4 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 unsigned int feature_persistent = 1;

    Use bool, please.

    > +module_param_named(feature_persistent, feature_persistent, int, 0644);

    module_param()

    > +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.
    > @@ -906,7 +912,8 @@ 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",
    > + feature_persistent ? 1 : 0);

    Using bool above should allow to just use the value of
    feature_persistent here.


    Juergen

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