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Re: [Xen-devel] Re: [PATCH] Support swap a page from user spacetools --

To: "Jiang, Yunhong" <yunhong.jiang@xxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxxxx>
Subject: Re: [Xen-devel] Re: [PATCH] Support swap a page from user spacetools -- Was RE: [RFC][PATCH] Basic support for page offline
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Fri, 20 Mar 2009 10:21:20 +0000
Cc: Tim Deegan <Tim.Deegan@xxxxxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
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Thread-topic: [Xen-devel] Re: [PATCH] Support swap a page from user spacetools -- Was RE: [RFC][PATCH] Basic support for page offline
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On 20/03/2009 10:13, "Jiang, Yunhong" <yunhong.jiang@xxxxxxxxx> wrote:

>> Even this may not work. Old grants will reference the old
>> page. Subsequent
>> attempts by a backend to map the grant will fail. And the
>> resulting failed
>> I/Os will probably make the frontend driver throw a fit. Getting this
>> working with suspend-cancel seems pretty tricky.
> If there is grant map for it, I think we will fail since the reference is not
> 1 when XENMEM_exchange.
> Or do you mean there is a reference in grant table but is not mapped still?
> Will the refrence count be added when a page is granted? I'm not quite sure
> about this, but I think that will be same to original XENMEM_exchange. If yes,
> we may have to update the grant information?

Reference counts are not adjusted until a grant is mapped. So I'm talking
about grants which have been written by the domU, passed to the backend
driver, but not yet used. Those will be screwed.

Anyway, see my email just now, which suggests you map the guest's grant
table into your toolstack code, and you can go check for existing grants
yourself. As I say there -- if you do that I don't think you need worry
about atomicity of the copy.

 -- Keir

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