On Tue, Sep 22, 2020 at 01:26:38PM +0200, SeongJae Park wrote: > On Tue, 22 Sep 2020 13:12:59 +0200 "Roger Pau Monné" <roger....@citrix.com> > wrote: > > > On Tue, Sep 22, 2020 at 12:52:07PM +0200, SeongJae Park wrote: > > > From: SeongJae Park <sjp...@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. 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 <aligu...@amazon.com> > > > Signed-off-by: SeongJae Park <sjp...@amazon.de> > > > --- > > > .../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 <sjp...@amazon.de> > > > +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.
If possible I prefer to avoid reading from xenstore because it's slow, very slow compared to storing this somewhere in the blkback structure. Could you use the feature_gnt_persistent field in xen_vbd maybe as a place to cache the value before it's set with the final negotiated value between the frontend and the backend? Thanks, Roger.
