linux-kernel - Re: [PATCH 04/14] mm,migration: Allow the migration of PageSwapCache pages

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Message-Id: <20100422085943.3d908a4b.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Thu, 22 Apr 2010 08:59:43 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	Mel Gorman <mel@....ul.ie>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Minchan Kim <minchan.kim@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 04/14] mm,migration: Allow the migration of
 PageSwapCache pages

On Wed, 21 Apr 2010 09:30:20 -0500 (CDT)
Christoph Lameter <cl@...ux-foundation.org> wrote:

> On Tue, 20 Apr 2010, Mel Gorman wrote:
> 
> > @@ -520,10 +521,12 @@ static int move_to_new_page(struct page *newpage, struct page *page)
> >  	else
> >  		rc = fallback_migrate_page(mapping, newpage, page);
> >
> > -	if (!rc)
> > -		remove_migration_ptes(page, newpage);
> > -	else
> > +	if (rc) {
> >  		newpage->mapping = NULL;
> > +	} else {
> > +		if (remap_swapcache)
> > +			remove_migration_ptes(page, newpage);
> > +	}
> 
> You are going to keep the migration ptes after the page has been unlocked?
> Or is remap_swapcache true if its not a swapcache page?
> 
> Maybe you meant
> 
> if (!remap_swapcache)
> 

Ah....Can I confirm my understanding ?

remap_swapcache == true only when
  The old page was ANON && it is not mapped. && it is SwapCache.

We do above check under lock_page(). So, this SwapCache is never mapped until
we release lock_page() on the old page. So, we don't use migration_pte in
this case because try_to_unmap() do nothing and don't need to call
remove_migration_pte().

If migration_pte is used somewhere...I think it's bug.

-Kame



> ?
> 
> >  	unlock_page(newpage);
> >
> 
> >
> >  skip_unmap:
> >  	if (!page_mapped(page))
> > -		rc = move_to_new_page(newpage, page);
> > +		rc = move_to_new_page(newpage, page, remap_swapcache);
> >
> > -	if (rc)
> > +	if (rc && remap_swapcache)
> >  		remove_migration_ptes(page, page);
> >  rcu_unlock:
> >
> 
> Looks like you meant !remap_swapcache
> 
> --
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