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Subject: Re: [PATCH v2 2/3] xen-blkfront: 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,
        pdurrant@amazon.co.uk, linux-block@vger.kernel.org,
        xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org
References: <20200922105209.5284-1-sjpark@amazon.com>
 <20200922105209.5284-3-sjpark@amazon.com>
From:   =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= <jgross@suse.com>
Message-ID: <fdbaf955-0b92-d356-2792-21b27ea1087d@suse.com>
Date:   Tue, 22 Sep 2020 14:11:32 +0200
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Precedence: bulk

On 22.09.20 12:52, 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.  It can be disabled from blkback side using a
> module parameter, 'feature_persistent'.  But, it is impossible from
> blkfront side.  For the reason, this commit adds a blkfront module
> parameter for disabling of the feature.
> 
> [1] https://wiki.xen.org/wiki/Xen_4.3_Block_Protocol_Scalability
> 
> Signed-off-by: SeongJae Park <sjpark@amazon.de>
> ---
>   .../ABI/testing/sysfs-driver-xen-blkfront     |  9 ++++++
>   drivers/block/xen-blkfront.c                  | 28 +++++++++++++------
>   2 files changed, 29 insertions(+), 8 deletions(-)
> 
> diff --git a/Documentation/ABI/testing/sysfs-driver-xen-blkfront b/Documentation/ABI/testing/sysfs-driver-xen-blkfront
> index c0a6cb7eb314..9c31334cb2e6 100644
> --- a/Documentation/ABI/testing/sysfs-driver-xen-blkfront
> +++ b/Documentation/ABI/testing/sysfs-driver-xen-blkfront
> @@ -8,3 +8,12 @@ Description:
>                   is 32 - higher value means more potential throughput but more
>                   memory usage. The backend picks the minimum of the frontend
>                   and its default backend value.
> +
> +What:           /sys/module/xen_blkfront/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 frontends.
> +                The default is Y (enable).
> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
> index 91de2e0755ae..49c324f377de 100644
> --- a/drivers/block/xen-blkfront.c
> +++ b/drivers/block/xen-blkfront.c
> @@ -149,6 +149,13 @@ static unsigned int xen_blkif_max_ring_order;
>   module_param_named(max_ring_page_order, xen_blkif_max_ring_order, int, 0444);
>   MODULE_PARM_DESC(max_ring_page_order, "Maximum order of pages to be used for the shared ring");
>   
> +/* 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");
> +
> +
>   #define BLK_RING_SIZE(info)	\
>   	__CONST_RING_SIZE(blkif, XEN_PAGE_SIZE * (info)->nr_ring_pages)
>   
> @@ -1866,11 +1873,13 @@ static int talk_to_blkback(struct xenbus_device *dev,
>   		message = "writing protocol";
>   		goto abort_transaction;
>   	}
> -	err = xenbus_printf(xbt, dev->nodename,
> -			    "feature-persistent", "%u", 1);
> -	if (err)
> -		dev_warn(&dev->dev,
> -			 "writing persistent grants feature to xenbus");
> +	if (feature_persistent) {
> +		err = xenbus_printf(xbt, dev->nodename,
> +				    "feature-persistent", "%u", 1);
> +		if (err)
> +			dev_warn(&dev->dev,
> +				 "writing persistent grants feature to xenbus");
> +	}
>   
>   	err = xenbus_transaction_end(xbt, 0);
>   	if (err) {
> @@ -2316,9 +2325,12 @@ static void blkfront_gather_backend_features(struct blkfront_info *info)
>   	if (xenbus_read_unsigned(info->xbdev->otherend, "feature-discard", 0))
>   		blkfront_setup_discard(info);
>   
> -	info->feature_persistent =
> -		!!xenbus_read_unsigned(info->xbdev->otherend,
> -				       "feature-persistent", 0);
> +	if (feature_persistent)
> +		info->feature_persistent =
> +			!!xenbus_read_unsigned(info->xbdev->otherend,
> +					       "feature-persistent", 0);
> +	else
> +		info->feature_persistent = 0;
>   
>   	indirect_segments = xenbus_read_unsigned(info->xbdev->otherend,
>   					"feature-max-indirect-segments", 0);
> 

Here you have the same problem as in blkback: feature_persistent could
change its value between the two tests.


Juergen