linux-kernel - Re: [PATCH] mm: implement WasActive page flag (for improving cleancache)

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Message-Id: <20120130175730.de654d9c.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Mon, 30 Jan 2012 17:57:30 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
Cc:	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Konrad Wilk <konrad.wilk@...cle.com>,
	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Nitin Gupta <ngupta@...are.org>,
	Nebojsa Trpkovic <trx.lists@...il.com>, minchan@...nel.org,
	Chris Mason <chris.mason@...cle.com>
Subject: Re: [PATCH] mm: implement WasActive page flag (for improving
 cleancache)

On Thu, 26 Jan 2012 21:15:16 -0800 (PST)
Dan Magenheimer <dan.magenheimer@...cle.com> wrote:

> > From: Rik van Riel [mailto:riel@...hat.com]
> > Subject: Re: [PATCH] mm: implement WasActive page flag (for improving cleancache)
> > 
> > On 01/26/2012 09:43 PM, Dan Magenheimer wrote:
> > 
> > > Maybe the Active page bit could be overloaded with some minor
> > > rewriting?  IOW, perhaps the Active bit could be ignored when
> > > the page is moved to the inactive LRU?  (Confusing I know, but I am
> > > just brainstorming...)
> > 
> > The PG_referenced bit is already overloaded.  We keep
> > the bit set when we move a page from the active to the
> > inactive list, so a page that was previously active
> > only needs to be referenced once to become active again.
> > 
> > The LRU bits (PG_lru, PG_active, etc) are needed to
> > figure out which LRU list the page is on.  I don't
> > think we can overload those...
> 
> I suspected that was true, but was just brainstorming.
> Thanks for confirming.
> 
> Are there any other page bits that are dont-care when
> a page is on an LRU list?
> 
> I'd also be interested in your/RedHat's opinion on the
> 64-bit vs 32-bit market.  Will RHEL7 even support 32-bit?
> 

How about replacing PG_slab ? 

I think  PageSlab(page) be implemented as

#define SLABMAGIC		(some value)
#define PageSlab(page)		(page->mapping == SLABMAGIC) 

or some...

Thanks,
-Kame

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