linux-kernel - RE: Subject: [PATCH V6 2/4] mm: frontswap: core code

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Date:	Tue, 9 Aug 2011 08:03:52 -0700 (PDT)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	Jan Beulich <JBeulich@...ell.com>
Cc:	hannes@...xchg.org, jackdachef@...il.com, hughd@...gle.com,
	jeremy@...p.org, npiggin@...nel.dk, linux-mm@...ck.org,
	akpm@...ux-foundation.org, sjenning@...ux.vnet.ibm.com,
	Chris Mason <chris.mason@...cle.com>,
	Konrad Wilk <konrad.wilk@...cle.com>,
	Kurt Hackel <kurt.hackel@...cle.com>, riel@...hat.com,
	ngupta@...are.org, linux-kernel@...r.kernel.org, matthew@....cx
Subject: RE: Subject: [PATCH V6 2/4] mm: frontswap: core code

> > +#ifndef CONFIG_FRONTSWAP
> > +/* all inline routines become no-ops and all externs are ignored */
> > +#define frontswap_enabled (0)
> > +#endif
> > +
> > +static inline int frontswap_test(struct swap_info_struct *sis, pgoff_t
> > offset)
> > +{
> > +	int ret = 0;
> > +
> > +	if (frontswap_enabled && sis->frontswap_map)
> > +		ret = test_bit(offset % BITS_PER_LONG,
> > +			&sis->frontswap_map[offset/BITS_PER_LONG]);
> 
> 	if (sis->frontswap_map)
> 		ret = test_bit(offset, sis->frontswap_map);
> 
> (since sis->frontswap_map can't be non-NULL without
> frontswap_enabled being true, and since test_bit() itself already
> does what you open-coded here.

Hi Jan --

Thanks for the review!

> since test_bit() itself already does what you open-coded here

Good catch.  Will change.  Either is correct and I suspect the
compiler may end up generating the same code, but your code
is much more succinct.

> (since sis->frontswap_map can't be non-NULL without
> frontswap_enabled being true

As noted in the comment immediately preceding, the frontswap_enabled
check serves a second purpose:  When CONFIG_FRONTSWAP is
disabled, this entire inline function devolves to a compile-time
constant (0), which avoids a handful of ifdef's in the
core swap subsystem.  (This approach was originally suggested
for cleancache by Jeremy Fitzhardinge.)

Also, though this patch never unsets frontswap_enabled, it is
a global and some future tmem backend might unset it, so
it's probably best to leave the extra test anyway.
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