linux-kernel - Re: [PATCHv2] rdma: add a new IB_ACCESS

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Date:	Wed, 10 Apr 2013 00:32:31 -0400
From:	"Michael R. Hines" <mrhines@...ux.vnet.ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	Roland Dreier <roland@...nel.org>,
	Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
	Sean Hefty <sean.hefty@...el.com>,
	Hal Rosenstock <hal.rosenstock@...il.com>,
	Yishai Hadas <yishaih@...lanox.com>,
	Christoph Lameter <cl@...ux.com>,
	"linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>, qemu-devel@...gnu.org
Subject: Re: [PATCHv2] rdma: add a new IB_ACCESS_GIFT flag

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@...ux.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.
>>>>>

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