Re: Frontswap [PATCH 0/4] (was Transcendent Memory): overview

[Avi Kivity <avi@redhat.com>]

On 04/25/2010 03:41 AM, Dan Magenheimer wrote:
>>> No, ANY put_page can fail, and this is a critical part of the API
>>> that provides all of the flexibility for the hypervisor and all
>>> the guests. (See previous reply.)
>>>        
>> The guest isn't required to do any put_page()s.  It can issue lots of
>> them when memory is available, and keep them in the hypervisor forever.
>> Failing new put_page()s isn't enough for a dynamic system, you need to
>> be able to force the guest to give up some of its tmem.
>>      
> Yes, indeed, this is true.  That is why it is important for any
> policy implemented behind frontswap to "bill" the guest if it
> is attempting to keep frontswap pages in the hypervisor forever
> and to prod the guest to reclaim them when it no longer needs
> super-fast emergency swap space.  The frontswap patch already includes
> the kernel mechanism to enable this and the prodding can be implemented
> by a guest daemon (of which there already exists an existence proof).
>    

In this case you could use the same mechanism to stop new put_page()s?

Seems frontswap is like a reverse balloon, where the balloon is in 
hypervisor space instead of the guest space.

> (While devil's advocacy is always welcome, frontswap is NOT a
> cool academic science project where these issues have not been
> considered or tested.)
>    


Good to know.

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