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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>,
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        <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v3 1/3] xen-blkback: add a parameter for disabling of persistent grants
Date:   Tue, 22 Sep 2020 16:30:11 +0200
Message-ID: <20200922143011.29568-1-sjpark@amazon.com>
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On Tue, 22 Sep 2020 16:25:57 +0200 "Jürgen Groß" <jgross@suse.com> wrote:

> On 22.09.20 16:15, 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 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>
> > ---
> >   .../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..f4c8827fa0ad 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 ? 1 : 0;
> 
> Just assign the value instead of using the ternary operator?

I will do so in the next version.

> 
> With that changed you can add my
> 
> Reviewed-by: Juergen Gross <jgross@suse.com>

Thank you for your kind, quick, and detailed review!


Thanks,
SeongJae Park