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

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