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SubjectRe: [PATCH v2 2/3] xen-blkfront: add a parameter for disabling of persistent grants
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On Tue, 22 Sep 2020 14:11:32 +0200 "Jürgen Groß" <jgross@suse.com> wrote:

> 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/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.

Yes, indeed. I will fix this in the next version.


Thanks,
SeongJae Park

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