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

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Message-ID: <20090819132420.GA6137@localhost>
Date:	Wed, 19 Aug 2009 21:24:20 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.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?

On Wed, Aug 19, 2009 at 08:25:56PM +0800, Minchan Kim wrote:
> 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.
> 
> 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 ?

No, IMHO your patch is simple and good, while KOSAKI's is more
complete :)

- the try_set_page_mlocked() rename is suitable
- the call to try_set_page_mlocked() is necessary on try_to_unmap()
- the "if (VM_LOCKED) referenced = 0" in page_referenced() could
  cover both active/inactive vmscan

I did like your proposed

                if (sc->order <= PAGE_ALLOC_COSTLY_ORDER &&
-                                       referenced && page_mapping_inuse(page))
+                                       referenced && page_mapping_inuse(page)
+                                       && !(vm_flags & VM_LOCKED))
                        goto activate_locked;

which looks more intuitive and less confusing.

Thanks,
Fengguang
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