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From:   SeongJae Park <sjpark@amazon.com>
To:     =?UTF-8?q?J=C3=BCrgen=20Gro=C3=9F?= <jgross@suse.com>
CC:     SeongJae Park <sjpark@amazon.com>, <konrad.wilk@oracle.com>,
        <roger.pau@citrix.com>, SeongJae Park <sjpark@amazon.de>,
        <axboe@kernel.dk>, <aliguori@amazon.com>, <amit@kernel.org>,
        <mheyne@amazon.de>, <linux-block@vger.kernel.org>,
        <xen-devel@lists.xenproject.org>, <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] xen-blkback: add a parameter for disabling of persistent grants
Date:   Tue, 22 Sep 2020 09:24:37 +0200
Message-ID: <20200922072437.2495-1-sjpark@amazon.com>
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On Tue, 22 Sep 2020 09:18:05 +0200 "Jürgen Groß" <jgross@suse.com> wrote:

> 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.

Oops, I will.

> 
> > +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.

Indeed.  I will fix these as you recommended in the next spin.


Thanks,
SeongJae Park