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Date: Mon, 30 Jan 2012 17:57:30 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
To: Dan Magenheimer <dan.magenheimer@oracle.com>
Cc: Rik van Riel <riel@redhat.com>, Andrew Morton <akpm@linux-foundation.org>,
        Dave Hansen <dave@linux.vnet.ibm.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, Konrad Wilk <konrad.wilk@oracle.com>,
        Seth Jennings <sjenning@linux.vnet.ibm.com>,
        Nitin Gupta <ngupta@vflare.org>,
        Nebojsa Trpkovic <trx.lists@gmail.com>, minchan@kernel.org,
        Chris Mason <chris.mason@oracle.com>
Subject: Re: [PATCH] mm: implement WasActive page flag (for improving
 cleancache)
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On Thu, 26 Jan 2012 21:15:16 -0800 (PST)
Dan Magenheimer <dan.magenheimer@oracle.com> wrote:

> > From: Rik van Riel [mailto:riel@redhat.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