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

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Date:	Wed, 19 Aug 2009 22:19:46 +0900
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	Wu Fengguang <fengguang.wu@...el.com>,
	Rik van Riel <riel@...hat.com>,
	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>,
	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?

2009/8/19 Minchan Kim <minchan.kim@...il.com>:
> On Wed, Aug 19, 2009 at 9:10 PM, Wu Fengguang<fengguang.wu@...el.com> wrote:
>> On Wed, Aug 19, 2009 at 08:05:19PM +0800, KOSAKI Motohiro wrote:
>>> >> page_referenced_file?
>>> >> I think we should change page_referenced().
>>> >
>>> > Yeah, good catch.
>>> >
>>> >>
>>> >> Instead, How about this?
>>> >> ==============================================
>>> >>
>>> >> Subject: [PATCH] mm: stop circulating of referenced mlocked pages
>>> >>
>>> >> Currently, mlock() systemcall doesn't gurantee to mark the page PG_Mlocked
>>> >
>>> >                                                    mark PG_mlocked
>>> >
>>> >> because some race prevent page grabbing.
>>> >> In that case, instead vmscan move the page to unevictable lru.
>>> >>
>>> >> However, Recently Wu Fengguang pointed out current vmscan logic isn't so
>>> >> efficient.
>>> >> mlocked page can move circulatly active and inactive list because
>>> >> vmscan check the page is referenced _before_ cull mlocked page.
>>> >>
>>> >> Plus, vmscan should mark PG_Mlocked when cull mlocked page.
>>> >
>>> >                           PG_mlocked
>>> >
>>> >> Otherwise vm stastics show strange number.
>>> >>
>>> >> This patch does that.
>>> >
>>> > Reviewed-by: Wu Fengguang <fengguang.wu@...el.com>
>>>
>>> Thanks.
>>>
>>>
>>>
>>> >> Index: b/mm/rmap.c
>>> >> ===================================================================
>>> >> --- a/mm/rmap.c       2009-08-18 19:48:14.000000000 +0900
>>> >> +++ b/mm/rmap.c       2009-08-18 23:47:34.000000000 +0900
>>> >> @@ -362,7 +362,9 @@ static int page_referenced_one(struct pa
>>> >>        * unevictable list.
>>> >>        */
>>> >>       if (vma->vm_flags & VM_LOCKED) {
>>> >> -             *mapcount = 1;  /* break early from loop */
>>> >> +             *mapcount = 1;          /* break early from loop */
>>> >> +             *vm_flags |= VM_LOCKED; /* for prevent to move active list */
>>> >
>>> >> +             try_set_page_mlocked(vma, page);
>>> >
>>> > That call is not absolutely necessary?
>>>
>>> Why? I haven't catch your point.
>>
>> Because we'll eventually hit another try_set_page_mlocked() when
>> trying to unmap the page. Ie. duplicated with another call you added
>> in this patch.

Correct.


> Yes. we don't have to call it and we can make patch simple.
> I already sent patch on yesterday.
>
> http://marc.info/?l=linux-mm&m=125059325722370&w=2
>
> I think It's more simple than KOSAKI's idea.
> Is any problem in my patch ?

Hmm, I think

1. Anyway, we need turn on PG_mlock.
2. PG_mlock prevent livelock because page_evictable() check is called
at very early in shrink_page_list().
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