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

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Date:	Tue, 27 Apr 2010 20:13:39 +0530
From:	Nitin Gupta <ngupta@...are.org>
To:	Pavel Machek <pavel@....cz>
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 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.

So, how frontswap approach can be seen as introducing a new API?

Thanks,
Nitin






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