linux-kernel - RE: Subject: [PATCH V6 1/4] mm: frontswap: swap data structure changes

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Message-Id: <4E4179D90200007800050676@nat28.tlf.novell.com>
Date:	Tue, 09 Aug 2011 17:18:01 +0100
From:	"Jan Beulich" <JBeulich@...ell.com>
To:	"Dan Magenheimer" <dan.magenheimer@...cle.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 1/4] mm: frontswap: swap data structure
	 changes

>>> On 09.08.11 at 17:03, Dan Magenheimer <dan.magenheimer@...cle.com> wrote:
>> > --- linux/include/linux/swap.h	2011-08-08 08:19:25.880690134 -0600
>> > +++ frontswap/include/linux/swap.h	2011-08-08 08:59:03.952691415 -0600
>> > @@ -194,6 +194,8 @@ struct swap_info_struct {
>> >  	struct block_device *bdev;	/* swap device or bdev of swap file */
>> >  	struct file *swap_file;		/* seldom referenced */
>> >  	unsigned int old_block_size;	/* seldom referenced */
>> 
>> #ifdef CONFIG_FRONTSWAP
>> 
>> > +	unsigned long *frontswap_map;	/* frontswap in-use, one bit per page */
>> > +	unsigned int frontswap_pages;	/* frontswap pages in-use counter */
>> 
>> 
>> #endif
>> 
>> (to eliminate any overhead with that config option unset)
>> 
>> Jan
> 
> Hi Jan --
> 
> Thanks for the review!
> 
> As noted in the commit comment, if these structure elements are
> not put inside an #ifdef CONFIG_FRONTSWAP, it becomes
> unnecessary to clutter the core swap code with several ifdefs.
> The cost is one pointer and one unsigned int per allocated
> swap device (often no more than one swap device per system),
> so the code clarity seemed more important than the tiny
> additional runtime space cost.
> 
> Do you disagree?

Not necessarily - I just know that in other similar occasions (partly
internally to our company) I was asked to make sure turned off
features would not leave *any* run time foot print whatsoever.

Jan

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