Re: [PATCHv2] rdma: add a new IB_ACCESS_GIFT flag

Michael R. Hines
2013-04-10T04:32:40Z

On 04/09/2013 11:24 PM, Michael S. Tsirkin wrote:
> Which mechanism do you refer to? You patches still seem to pin each
> page in guest memory at some point, which will break all COW. In
> particular any pagemap tricks to detect duplicates on source that I
> suggested won't work.

Sorry, I mispoke. I'm reffering to dynamic server page registration.

Of course it does not eliminate pinning - but it does mitigate the foot
print of the VM as a feature that was requested.

I have implemented it and documented it.

- Michael

>> On 04/09/2013 03:03 PM, Michael S. Tsirkin wrote:
>>> presumably is_dup_page reads the page, so should not break COW ...
>>>
>>> I'm not sure about the cgroups swap limit - you might have
>>> too many non COW pages so attempting to fault them all in
>>> makes you exceed the limit. You really should look at
>>> what is going on in the pagemap, to see if there's
>>> measureable gain from the patch.
>>>
>>>
>>> On Fri, Apr 05, 2013 at 05:32:30PM -0400, Michael R. Hines wrote:
>>>> Well, I have the "is_dup_page()" commented out.......when RDMA is
>>>> activated.....
>>>>
>>>> Is there something else in QEMU that could be touching the page that
>>>> I don't know about?
>>>>
>>>> - Michael
>>>>
>>>>
>>>> On 04/05/2013 05:03 PM, Roland Dreier wrote:
>>>>> On Fri, Apr 5, 2013 at 1:51 PM, Michael R. Hines
>>>>> <mrhines@linux.vnet.ibm.com> wrote:
>>>>>> Sorry, I was wrong. ignore the comments about cgroups. That's still broken.
>>>>>> (i.e. trying to register RDMA memory while using a cgroup swap limit cause
>>>>>> the process get killed).
>>>>>>
>>>>>> But the GIFT flag patch works (my understanding is that GIFT flag allows the
>>>>>> adapter to transmit stale memory information, it does not have anything to
>>>>>> do with cgroups specifically).
>>>>> The point of the GIFT patch is to avoid triggering copy-on-write so
>>>>> that memory doesn't blow up during migration. If that doesn't work
>>>>> then there's no point to the patch.
>>>>>
>>>>> - R.
>>>>>