Re: [PATCH v2 2/3] xen-blkfront: add a parameter for disabling of persistent grants

[Jürgen Groß <jgross@xxxxxxxx>]

On 22.09.20 12:52, SeongJae Park wrote:
> From: SeongJae Park <sjpark@xxxxxxxxx>
>
> 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@xxxxxxxxx>
> ---
>   .../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@xxxxxxxxx>
> +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