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

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Message-ID: <20100429130411.GB1661@ucw.cz>
Date:	Thu, 29 Apr 2010 15:04:12 +0200
From:	Pavel Machek <pavel@....cz>
To:	Nitin Gupta <ngupta@...are.org>
Cc:	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Avi Kivity <avi@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, jeremy@...p.org, hugh.dickins@...cali.co.uk,
	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

On Tue 2010-04-27 20:13:39, Nitin Gupta wrote:
> On 04/27/2010 06:25 PM, Pavel Machek wrote:
> > 
> >>> Can we extend it?  Adding new APIs is easy, but harder to maintain in
> >>> the long term.
> >>
> >> Umm... I think the difference between a "new" API and extending
> >> an existing one here is a choice of semantics.  As designed, frontswap
> >> is an extremely simple, only-very-slightly-intrusive set of hooks that
> >> allows swap pages to, under some conditions, go to pseudo-RAM instead
> > ...
> >> "Extending" the existing swap API, which has largely been untouched for
> >> many years, seems like a significantly more complex and error-prone
> >> undertaking that will affect nearly all Linux users with a likely long
> >> bug tail.  And, by the way, there is no existence proof that it
> >> will be useful.
> > 
> >> Seems like a no-brainer to me.
> > 
> > Stop right here. Instead of improving existing swap api, you just
> > create one because it is less work.
> > 
> > We do not want apis to cummulate; please just fix the existing one.
> 
> 
> I'm a bit confused: What do you mean by 'existing swap API'?
> Frontswap simply hooks in swap_readpage() and swap_writepage() to
> call frontswap_{get,put}_page() respectively. Now to avoid a hardcoded
> implementation of these function, it introduces struct frontswap_ops
> so that custom implementations fronswap get/put/etc. functions can be
> provided. This allows easy implementation of swap-to-hypervisor,
> in-memory-compressed-swapping etc. with common set of hooks.

Yes, and that set of hooks is new API, right?

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
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