linux-kernel - Re: [PATCH 2/5] mm: frontswap: lazy initialization to allow tmem backends to build/run as modules

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Message-ID: <20121102182429.GA30100@konrad-lan.dumpdata.com>
Date:	Fri, 2 Nov 2012 14:24:30 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
Cc:	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	devel@...uxdriverproject.org, linux-kernel@...r.kernel.org,
	gregkh@...uxfoundation.org, linux-mm@...ck.org, ngupta@...are.org,
	minchan@...nel.org, fschmaus@...il.com, andor.daam@...glemail.com,
	ilendir@...glemail.com, akpm@...ux-foundation.org, mgorman@...e.de
Subject: Re: [PATCH 2/5] mm: frontswap: lazy initialization to allow tmem
 backends to build/run as modules

> > > +	frontswap_enabled = 1;
> > 
> > If frontswap_enabled is going to be on all the time, then what point
> > does it serve?  By extension, can all of the static inline wrappers in
> > frontswap.h be done away with?

Hm, or the frontswap_enabled can be converted to a "frontswap_flag"
which has:

#define FRONTSWAP_ON (1<<1)
#define FRONTSWAP_BACKEND_ON (1<<2)

or so? And then we can see if we can squash the 'backend_registerd'
and 'frontswap_enabled' together.
> 
> The intent of frontswap_enabled and cleancache_enabled was
> to avoid the overhead of a function call at the point where
> each frontswap/cleancache "hooks" is placed, using a global
> variable check instead.  I'm not sure if this minor
> performance tuning effort is worth preserving:  If not,
> I agree frontswap_enabled and the static inline wrappers (as
> well as their cleancache brethren) could be done away with **;
> if worth preserving, then I think frontswap_enabled could
> be set in the init method instead but the check for enabled
> in the frontswap init method and the cleancache init_fs
> method would need to be removed else lazy initialization
> wouldn't work.

Either way, that should be a seperate patch.
> 
> Dan
> 
> ** Note to anyone that tries this:  There is a subtle but
> clever hack in the wrappers suggested by Jeremy Fitzhardinge
> that disables the wrappers at compile-time as well as
> runtime.  IOW, make sure you test-compile both with
> CONFIG_{CLEANCACHE|FRONTSWAP} _and_ with them unconfig'd.
> 
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