Re: [kvm-devel] [PATCH] export notifier #1

[Avi Kivity <avi@qumranet.com>]

Christoph Lameter wrote:
> Ahhh. Good to hear. But we will still end in a situation where only
> the remote ptes point to the page. Maybe the remote instance will dirty
> the page at that point?
>
>   

When the spte is dropped, its dirty bit is transferred to the page.
>   
>> sharing code, and for you missing a single notifier means memory
>> corruption because you don't bump the page count to represent the
>> external reference).
>>     
>
> The approach with the export notifier is page based not based on the 
> mm_struct. We only need a single page count for a page that is exported to 
> a number of remote instances of linux. The page count is dropped when all 
> the remote instances have unmapped the page.
>   

That won't work for kvm.  If we have a hundred virtual machines, that 
means 99 no-op notifications.

Also, our rmap key for finding the spte is keyed on (mm, va).  I imagine 
most RDMA cards are similar.
>  
>   
>>> @@ -966,6 +973,9 @@ int try_to_unmap(struct page *page, int 
>>>  
>>>  	BUG_ON(!PageLocked(page));
>>>  
>>> +	if (unlikely(PageExported(page)))
>>> +		export_notifier(invalidate_page, page);
>>> +
>>>       
>> Passing the page here will complicate things especially for shared
>> pages across different VM that are already working in KVM. For non
>>     
>
> How?
>
>   
>> shared pages we could cache the userland mapping address in
>> page->private but it's a kludge only working for non-shared
>> pages. Walking twice the anon_vma lists when only a single walk is
>>     
>
> There is only the need to walk twice for pages that are marked Exported. 
> And the double walk is only necessary if the exporter does not have its 
> own rmap. The cross partition thing that we are doing has such an rmap and 
> its a matter of walking the exporters rmap to clear out the external 
> references and then we walk the local rmaps. All once.
>   

The problem is that external mmus need a reverse mapping structure to 
locate their ptes.  We can't expand struct page so we need to base it on 
mm + va.

>   
>> Besides the pinned pages ram leak by having the zero locking window
>> above I'm curious how you are going to take care of the finegrined
>> aging that I'm doing with the accessed bit set by hardware in the spte
>>     
>
> I think I explained that above. Remote users effectively are forbidden to 
> establish new references to the page by the clearing of the exported bit.
>
>   

Can they wait on that bit?


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