Re: [PATCH v2 1/3] xen-blkback: add a parameter for disabling of persistent grants

From: SeongJae Park
Date: Tue Sep 22 2020 - 07:27:09 EST


On Tue, 22 Sep 2020 13:12:59 +0200 "Roger Pau Monné" <roger.pau@xxxxxxxxxx> wrote:

> On Tue, Sep 22, 2020 at 12:52:07PM +0200, 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. 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@xxxxxxxxxx>
> > Signed-off-by: SeongJae Park <sjpark@xxxxxxxxx>
> > ---
> > .../ABI/testing/sysfs-driver-xen-blkback | 9 ++++++
> > drivers/block/xen-blkback/xenbus.c | 28 ++++++++++++++-----
> > 2 files changed, 30 insertions(+), 7 deletions(-)
> >
> > diff --git a/Documentation/ABI/testing/sysfs-driver-xen-blkback b/Documentation/ABI/testing/sysfs-driver-xen-blkback
> > index ecb7942ff146..ac2947b98950 100644
> > --- a/Documentation/ABI/testing/sysfs-driver-xen-blkback
> > +++ b/Documentation/ABI/testing/sysfs-driver-xen-blkback
> > @@ -35,3 +35,12 @@ 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@xxxxxxxxx>
> > +Description:
> > + Whether to enable the persistent grants feature or not. Note
> > + that this option only takes effect on newly created backends.
> > + The default is Y (enable).
> > diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
> > index b9aa5d1ac10b..8a95ddd08b13 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 bool feature_persistent = true;
> > +module_param(feature_persistent, bool, 0644);
> > +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.
> > @@ -906,11 +912,15 @@ 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);
> > - if (err) {
> > - xenbus_dev_fatal(dev, err, "writing %s/feature-persistent",
> > - dev->nodename);
> > - goto abort;
> > + if (feature_persistent) {
> > + err = xenbus_printf(xbt, dev->nodename, "feature-persistent",
> > + "%u", feature_persistent);
> > + if (err) {
> > + xenbus_dev_fatal(dev, err,
> > + "writing %s/feature-persistent",
> > + dev->nodename);
> > + goto abort;
> > + }
> > }
> >
> > err = xenbus_printf(xbt, dev->nodename, "sectors", "%llu",
> > @@ -1093,8 +1103,12 @@ static int connect_ring(struct backend_info *be)
> > xenbus_dev_fatal(dev, err, "unknown fe protocol %s", protocol);
> > return -ENOSYS;
> > }
> > - pers_grants = xenbus_read_unsigned(dev->otherend, "feature-persistent",
> > - 0);
> > + if (feature_persistent)
> > + pers_grants = xenbus_read_unsigned(dev->otherend,
> > + "feature-persistent", 0);
> > + else
> > + pers_grants = 0;
> > +
>
> Sorry for not realizing earlier, but looking at it again I think you
> need to cache the value of feature_persistent when it's first used in
> the blkback state data, so that it's consistent.
>
> What would happen for example with the following flow (assume a
> persistent grants enabled frontend):
>
> feature_persistent = false
>
> connect(...)
> feature-persistent is not written to xenstore
>
> User changes feature_persistent = true
>
> connect_ring(...)
> pers_grants = true, because feature-persistent is set unconditionally
> by the frontend and feature_persistent variable is now true.
>
> Then blkback will try to use persistent grants and the whole
> connection will malfunction because the frontend won't.

Ah, you're right. I should also catch this before but didn't, sorry.

>
> The other option is to prevent changing the variable when there are
> blkback instances already running.

I think storing the option value in xenstore would be simpler. That said, if
you prefer this way, please let me know.


Thanks,
SeongJae Park