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

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Message-ID: <4F218D36.2060308@linux.vnet.ibm.com>
Date:	Thu, 26 Jan 2012 09:28:22 -0800
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
CC:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.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,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	riel@...hat.com, Chris Mason <chris.mason@...cle.com>
Subject: Re: [PATCH] mm: implement WasActive page flag (for improving cleancache)

On 01/25/2012 01:58 PM, Dan Magenheimer wrote:
> (Feedback welcome if there is a different/better way to do this
> without using a page flag!)
> 
> Since about 2.6.27, the page replacement algorithm maintains
> an "active" bit to help decide which pages are most eligible
> to reclaim, see http://linux-mm.org/PageReplacementDesign 
> 
> This "active' information is also useful to cleancache but is lost
> by the time that cleancache has the opportunity to preserve the
> pageful of data.  This patch adds a new page flag "WasActive" to
> retain the state.  The flag may possibly be useful elsewhere.

I guess cleancache itself is clearing the bit, right?  I didn't see any
clearing going on in the patch.

I do think it also needs to get cleared on the way in to the page
allocator.  Otherwise:

	PageSetWasActive(page);
	free_page(page);
	...
	another_user_page = get_free_page()
	// now cleancache sees the active bit for the prev user

Or am I missing somewhere it gets cleared non-explicitly somewhere?

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