On Tue, 22 Sep 2020 13:35:30 +0200 "Jürgen Groß" <jgross@xxxxxxxx> wrote: > On 22.09.20 13:26, SeongJae Park wrote: > > 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. > > No, Xenstore isn't the right place for that. This is a local > implementation detail of blkback and shouldn't be exported to Xenstore. Agreed. I will try using the 'feature_gnt_persistent' as Roger recommended. Thanks, SeongJae Park
