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