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

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Date:	Thu, 29 Apr 2010 21:59:14 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
CC:	Pavel Machek <pavel@....cz>, 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

On 04/29/2010 05:42 PM, Dan Magenheimer wrote:
>>
>> Yes, and that set of hooks is new API, right?
>>      
> Well, no, if you define API as "application programming interface"
> this is NOT exposed to userland.  If you define API as a new
> in-kernel function call, yes, these hooks are a new API, but that
> is true of virtually any new code in the kernel.  If you define
> API as some new interface between the kernel and a hypervisor,
> yes, this is a new API, but it is "optional" at several levels
> so that any hypervisor (e.g. KVM) can completely ignore it.
>    

The concern is not with the hypervisor, but with Linux.  More external 
APIs reduce our flexibility to change things.

> So please let's not argue about whether the code is a "new API"
> or not, but instead consider whether the concept is useful or not
> and if useful, if there is or is not a cleaner way to implement it.
>    

I'm convinced it's useful.  The API is so close to a block device 
(read/write with key/value vs read/write with sector/value) that we 
should make the effort not to introduce a new API.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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