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

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Message-Id: <20090818131734.3d5bceb2.minchan.kim@barrios-desktop>
Date:	Tue, 18 Aug 2009 13:17:34 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Minchan Kim <minchan.kim@...il.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>,
	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 Tue, 18 Aug 2009 10:34:38 +0800
Wu Fengguang <fengguang.wu@...el.com> wrote:

> Minchan,
> 
> On Mon, Aug 17, 2009 at 10:33:54PM +0800, Minchan Kim wrote:
> > On Sun, Aug 16, 2009 at 8:29 PM, Wu Fengguang<fengguang.wu@...el.com> wrote:
> > > 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?
> > 
> > I think it's not a big deal.
> 
> Maybe, otherwise I should bring up this issue long time before :)
> 
> > As you mentioned, it's rare case so there would be few pages in active
> > list instead of unevictable list.
> 
> Yes.
> 
> > When next time to scan comes, we can try to move the pages into
> > unevictable list, again.
> 
> Will PG_mlocked be set by then? Otherwise the situation is not likely 
> to change and the VM_LOCKED pages may circulate in active/inactive
> list for countless times.

PG_mlocked is not important in that case. 
Important thing is VM_LOCKED vma. 
I think below annotaion can help you to understand my point. :)

----

/*
 * called from munlock()/munmap() path with page supposedly on the LRU.
 *
 * Note:  unlike mlock_vma_page(), we can't just clear the PageMlocked
 * [in try_to_munlock()] and then attempt to isolate the page.  We must
 * isolate the page to keep others from messing with its unevictable
 * and mlocked state while trying to munlock.  However, we pre-clear the
 * mlocked state anyway as we might lose the isolation race and we might
 * not get another chance to clear PageMlocked.  If we successfully
 * isolate the page and try_to_munlock() detects other VM_LOCKED vmas
 * mapping the page, it will restore the PageMlocked state, unless the page
 * is mapped in a non-linear vma.  So, we go ahead and SetPageMlocked(),
 * perhaps redundantly.
 * If we lose the isolation race, and the page is mapped by other VM_LOCKED
 * vmas, we'll detect this in vmscan--via try_to_munlock() or try_to_unmap()
 * either of which will restore the PageMlocked state by calling
 * mlock_vma_page() above, if it can grab the vma's mmap sem.
 */
static void munlock_vma_page(struct page *page)
{
...

-- 
Kind regards,
Minchan Kim
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