Re: [Xen-devel] Re: [PATCH] Support swap a page from user spacetools -- Was RE: [RFC][PATCH] Basic support for page offline

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 20/03/2009 10:07, "Keir Fraser" <keir.fraser@xxxxxxxxxxxxx> wrote:

>>> Yes, that's suggested by Tim and I think that's meet our purpose quite well.
>> 
>> Okay, then I suggest you extend XENMEM_exchange so that in.mem_flags can
>> tell that hypercall to copy data from old to new pages. XENMEMF_copy_data?
> 
> 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.

In fact do you have *any* handling for grants which have not yet been used
by the backend? It seems that such grants are doomed to fail since you
probably don't rewrite them?

Would it be a good idea to map the guest's grant pages and scan for your
old_pfn? Then fail the exchange if you see a match? That avoids this whole
issue and then also you don't need to worry about racing backends and you
can keep the copy operation where it is, in the tools, I think.

 -- Keir



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