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Subject: Re: [PATCH] xen-blkback: add a parameter for disabling of persistent
 grants
To:     SeongJae Park <sjpark@amazon.com>, konrad.wilk@oracle.com,
        roger.pau@citrix.com
Cc:     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
References: <20200922070125.27251-1-sjpark@amazon.com>
From:   =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= <jgross@suse.com>
Message-ID: <5fd34475-c296-c4f3-2bac-180c166449fe@suse.com>
Date:   Tue, 22 Sep 2020 09:18:05 +0200
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Precedence: bulk

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