Re: [RFC] Expose infrastructure for unpinning guest memory

[Anthony Liguori <aliguori-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>]

Avi Kivity wrote:
> Anthony Liguori wrote:
>   
>> Now that we have userspace memory allocation, I wanted to play with ballooning.
>> The idea is that when a guest "balloons" down, we simply unpin the underlying
>> physical memory and the host kernel may or may not swap it.  To reclaim
>> ballooned memory, the guest can just start using it and we'll pin it on demand.
>>
>> The following patch is a stab at providing the right infrastructure for pinning
>> and automatic repinning.  I don't have a lot of comfort in the MMU code so I
>> thought I'd get some feedback before going much further.
>>
>> gpa_to_hpa is a little awkward to hook, but it seems like the right place in the
>> code.  I'm most uncertain about the SMP safety of the unpinning.  Presumably,
>> I have to hold the kvm lock around the mmu_unshadow and page_cache release to
>> ensure that another VCPU doesn't fault the page back in after mmu_unshadow?
>>
>>   
>>     
>
> One we have true swapping capabilities (which imply ability for the
> kernel to remove a page from the shadow page tables) you can unpin by
> calling munmap() or madvise(MADV_REMOVE) on the pages to be unpinned.
>   

So does MADV_REMOVE remove the backing page but still allow for memory 
to be faulted in?  That is, after calling MADV_REMOVE, there's no 
guarantee that the contents of a give VA range will remain the same (but 
it won't SEGV the app if it accesses that memory)?

If so, I think that would be the right way to treat it.  That allows for 
two types of hints for the guest to provide: 1) I won't access this 
memory for a very long time (so it's a good candidate to swap out) and 
2) I won't access this memory and don't care about it's contents.

Regards,

Anthony Liguori

> Other than that the approach seems right.
>
>   


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