linux-kernel - Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages

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Date:	Thu, 25 Mar 2010 18:41:23 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Mel Gorman <mel@....ul.ie>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped
 anonymous pages

On Thu, 25 Mar 2010 09:21:32 +0000
Mel Gorman <mel@....ul.ie> wrote:

> On Thu, Mar 25, 2010 at 06:09:34PM +0900, KOSAKI Motohiro wrote:
> > > On Thu, 25 Mar 2010 08:32:35 +0000
> > > Mel Gorman <mel@....ul.ie> wrote:
> > > 
> > > > On Thu, Mar 25, 2010 at 11:49:23AM +0900, KOSAKI Motohiro wrote:
> > > > > > On Fri, Mar 19, 2010 at 03:21:41PM +0900, KOSAKI Motohiro wrote: 
> > > > > Hmmm...
> > > > > I haven't understand your mention because I guess I was wrong.
> > > > > 
> > > > > probably my last question was unclear. I mean,
> > > > > 
> > > > > 1) If we still need SLAB_DESTROY_BY_RCU, why do we need to add refcount?
> > > > >     Which difference is exist between normal page migration and compaction?
> > > > 
> > > > The processes typically calling migration today own the page they are moving
> > > > and is not going to exit unexpectedly during migration.
> > > > 
> > > > > 2) If we added refcount, which race will solve?
> > > > > 
> > > > 
> > > > The process exiting and the last anon_vma being dropped while compaction
> > > > is running. This can be reliably triggered with compaction.
> > > > 
> > > > > IOW, Is this patch fix old issue or compaction specific issue?
> > > > > 
> > > > 
> > > > Strictly speaking, it's an old issue but in practice it's impossible to
> > > > trigger because the process migrating always owns the page. Compaction
> > > > moves pages belonging to arbitrary processes.
> > > > 
> > > Kosaki-san,
> > > 
> > >  IIUC, the race in memory-hotunplug was fixed by this patch [2/11].
> > > 
> > >  But, this behavior of unmap_and_move() requires access to _freed_
> > >  objects (spinlock). Even if it's safe because of SLAB_DESTROY_BY_RCU,
> > >  it't not good habit in general.
> > > 
> > >  After direct compaction, page-migration will be one of "core" code of
> > >  memory management. Then, I agree to patch [1/11] as our direction for
> > >  keeping sanity and showing direction to more updates. Maybe adding
> > >  refcnt and removing RCU in futuer is good.
> > 
> > But Christoph seems oppose to remove SLAB_DESTROY_BY_RCU. then refcount
> > is meaningless now.
> 
> Christoph is opposed to removing it because of cache-hotness issues more
> so than use-after-free concerns. The refcount is needed with or without
> SLAB_DESTROY_BY_RCU.
> 

I wonder a code which the easiest to be read will be like following.
==

        if (PageAnon(page)) {
                struct anon_vma anon = page_lock_anon_vma(page);
		/* to take this lock, this page must be mapped. */
		if (!anon_vma)
			goto uncharge;
		increase refcnt
		page_unlock_anon_vma(anon);
        }
	....
==
and
==
void anon_vma_free(struct anon_vma *anon)
{
	/*
	 * To increase refcnt of anon-vma, anon_vma->lock should be held by
	 * page_lock_anon_vma(). It means anon_vma has a "mapped" page.
	 * If this anon is freed by unmap or exit, all pages under this anon
	 * must be unmapped. Then, just checking refcnt without lock is ok.
	 */
	if (check refcnt > 0)
		return do nothing
	kmem_cache_free(anon);
}
==

Then, rcu_read_lock can be removed in clean way.

Thanks,
-Kame


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