linux-kernel - Re: [PATCH] mm,migration: Avoid race between shift_arg_pages() and rmap_walk() during migration by not migrating temporary stacks

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Message-Id: <20100514085135.cfb5ba17.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Fri, 14 May 2010 08:51:35 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Mel Gorman <mel@....ul.ie>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] mm,migration: Avoid race between shift_arg_pages() and
 rmap_walk() during migration by not migrating temporary stacks

On Thu, 13 May 2010 12:22:01 -0500 (CDT)
Christoph Lameter <cl@...ux.com> wrote:

> On Thu, 13 May 2010, KAMEZAWA Hiroyuki wrote:
> 
> > > Would it not be possible to do something similar for the temporary stack?
> > >
> >
> > Problem here is unmap->remap. ->migratepage() function is used as
> >
> > 	unmap
> > 	   -> migratepage()
> > 	      -> failed
> > 		-> remap
> >
> > Then, migratepage() itself is no help. We need some check-callback before unmap
> > or lock to wait for an event we can make remapping progress.
> 
> We could check earlier if the migrate function points to
> fail_migrate_page()?

If you mean checking before unmap, yes. we can avoid the race.
Unmapping itself is dangerous in this case.

> Where we check for PageKsm() in unmap_and_move f.e.?
> 

I'm not sure about KSM.

Thanks,
-Kame

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