Re: [kvm-devel] mmu notifiers

[Avi K. <av...@qu...>]

Christoph Lameter wrote:
> On Tue, 15 Jan 2008, Avi Kivity wrote:
>
>   
>>> But each guest has its own page structs. They cannot share page structs.
>>> Concurrent access of two independent kernel instances for synchronization
>>> and status maintenance to a single page struct?
>>>   
>>>       
>> There's a host page struct (that the guest know nothing about and cannot
>> touch), and optionally a guest page struct for each guest (that the host and
>> the other guest know nothing about).
>>     
>
> Ok so if two linux guests want to share memory three page structs are 
> involved:
>
> 1. Host page struct
> 2. Guest #1 page struct
> 3. Guest #2 page struct
>
> I can understand that 1 and 2 point to the same physical page. Even all 
> three could point to the same page if the page is readonly. 
>
> However, lets say that Guest #1 allocates some anonymous memory and wants
> to share it with Guest #2. In that case something like PFNMAP is likely
> going to be used? Or are you remapping the physical page so that #1 and #2 
> share it? In that case two page struct describe state of the same physical
> page and we have no effective synchronization for writeback etc.
>
>   

Like I said, out of the box Linux doesn't support using memory that is 
shared with other instances as main memory.  One usage  (by the s390 
folk) was to put a read-only filesystem with execute-in-place support on 
this memory, and so reduce the memory usage of guests.

>> The host page struct may disappear if the host decides to swap the page into
>> its backing store and free the page.  The guest page structs (if any) would
>> remain.
>>     
>
> Page structs never disappear. The pte's may disappear and the page may be 
> unmapped from an address space of a process but the page struct stays. 
> Page struct can only disappear if memory hotplug is activated and memory 
> is taken out of the system.
>   

Yes, that was poorly phrased.  The page and its page struct may be 
reallocated for other purposes.

-- 
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