linux-kernel - RE: Frontswap [PATCH 0/4] (was Transcendent Memory): overview

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Message-ID: <1c02a94a-a6aa-4cbb-a2e6-9d4647760e91@default>
Date:	Sat, 24 Apr 2010 17:41:04 -0700 (PDT)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org, jeremy@...p.org,
	hugh.dickins@...cali.co.uk, ngupta@...are.org, JBeulich@...ell.com,
	chris.mason@...cle.com, kurt.hackel@...cle.com,
	dave.mccracken@...cle.com, npiggin@...e.de,
	akpm@...ux-foundation.org, riel@...hat.com
Subject: RE: Frontswap [PATCH 0/4] (was Transcendent Memory): overview

> > 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).

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