linux-kernel - Re: [RFC] respect the referenced bit of KVM guest pages?

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Date:	Sun, 16 Aug 2009 13:15:02 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	Jeff Dike <jdike@...toit.com>, Avi Kivity <avi@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	"Yu, Wilfred" <wilfred.yu@...el.com>,
	"Kleen, Andi" <andi.kleen@...el.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Mel Gorman <mel@....ul.ie>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [RFC] respect the referenced bit of KVM guest pages?

On Sun, Aug 16, 2009 at 11:53:00AM +0800, Rik van Riel wrote:
> Wu Fengguang wrote:
> > On Fri, Aug 07, 2009 at 05:09:55AM +0800, Jeff Dike wrote:
> >> Side question -
> >> 	Is there a good reason for this to be in shrink_active_list()
> >> as opposed to __isolate_lru_page?
> >>
> >> 		if (unlikely(!page_evictable(page, NULL))) {
> >> 			putback_lru_page(page);
> >> 			continue;
> >> 		}
> >>
> >> Maybe we want to minimize the amount of code under the lru lock or
> >> avoid duplicate logic in the isolate_page functions.
> > 
> > I guess the quick test means to avoid the expensive page_referenced()
> > call that follows it. But that should be mostly one shot cost - the
> > unevictable pages are unlikely to cycle in active/inactive list again
> > and again.
> 
> Please read what putback_lru_page does.
> 
> It moves the page onto the unevictable list, so that
> it will not end up in this scan again.

Yes it does. I said 'mostly' because there is a small hole that an
unevictable page may be scanned but still not moved to unevictable
list: when a page is mapped in two places, the first pte has the
referenced bit set, the _second_ VMA has VM_LOCKED bit set, then
page_referenced() will return 1 and shrink_page_list() will move it
into active list instead of unevictable list. Shall we fix this rare
case?

> >> But if there are important mlock-heavy workloads, this could make the
> >> scan come up empty, or at least emptier than we might like.
> > 
> > Yes, if the above 'if' block is removed, the inactive lists might get
> > more expensive to reclaim.
> 
> Why?

Without the 'if' block, an unevictable page may well be deactivated into
inactive list (and some time later be moved to unevictable list
from there), increasing the inactive list's scanned:reclaimed ratio.

Thanks,
Fengguang
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