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

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Message-ID: <20090816112910.GA3208@localhost>
Date:	Sun, 16 Aug 2009 19:29:10 +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 01:15:02PM +0800, Wu Fengguang wrote:
> 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?

How about this fix?

---
mm: stop circulating of referenced mlocked pages

Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
---

--- linux.orig/mm/rmap.c	2009-08-16 19:11:13.000000000 +0800
+++ linux/mm/rmap.c	2009-08-16 19:22:46.000000000 +0800
@@ -358,6 +358,7 @@ static int page_referenced_one(struct pa
 	 */
 	if (vma->vm_flags & VM_LOCKED) {
 		*mapcount = 1;	/* break early from loop */
+		*vm_flags |= VM_LOCKED;
 		goto out_unmap;
 	}
 
@@ -482,6 +483,8 @@ static int page_referenced_file(struct p
 	}
 
 	spin_unlock(&mapping->i_mmap_lock);
+	if (*vm_flags & VM_LOCKED)
+		referenced = 0;
 	return referenced;
 }
 
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