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

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Date:	Thu, 1 Nov 2012 08:30:05 -0700 (PDT)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	Seth Jennings <sjenning@...ux.vnet.ibm.com>
Cc:	devel@...uxdriverproject.org, linux-kernel@...r.kernel.org,
	gregkh@...uxfoundation.org, linux-mm@...ck.org, ngupta@...are.org,
	Konrad Wilk <konrad.wilk@...cle.com>, 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

> From: Seth Jennings [mailto:sjenning@...ux.vnet.ibm.com]
> Subject: Re: [PATCH 2/5] mm: frontswap: lazy initialization to allow tmem backends to build/run as
> modules
> 
> >  static int __init init_frontswap(void)
> >  {
> > +	int i;
> >  #ifdef CONFIG_DEBUG_FS
> >  	struct dentry *root = debugfs_create_dir("frontswap", NULL);
> >  	if (root == NULL)
> > @@ -364,6 +414,10 @@ static int __init init_frontswap(void)
> >  	debugfs_create_u64("invalidates", S_IRUGO,
> >  				root, &frontswap_invalidates);
> >  #endif
> > +	for (i = 0; i < MAX_INITIALIZABLE_SD; i++)
> > +		sds[i] = -1;
> > +
> > +	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?

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.

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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