Messages in this thread | | | Subject | Re: [PATCH v2 1/3] xen-blkback: add a parameter for disabling of persistent grants | From | Jürgen Groß <> | Date | Tue, 22 Sep 2020 13:35:30 +0200 |
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On 22.09.20 13:26, SeongJae Park wrote: > On Tue, 22 Sep 2020 13:12:59 +0200 "Roger Pau Monné" <roger.pau@citrix.com> wrote: > >> On Tue, Sep 22, 2020 at 12:52:07PM +0200, 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. 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@amazon.com> >>> Signed-off-by: SeongJae Park <sjpark@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 <sjpark@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.
No, Xenstore isn't the right place for that. This is a local implementation detail of blkback and shouldn't be exported to Xenstore.
Juergen
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