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

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Date:	Mon, 15 Mar 2010 16:09:19 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Minchan Kim <minchan.kim@...il.com>, Mel Gorman <mel@....ul.ie>,
	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>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.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 Mon, 15 Mar 2010 15:44:59 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:

> On Mon, 15 Mar 2010 15:28:15 +0900
> Minchan Kim <minchan.kim@...il.com> wrote:
> 
> > On Mon, Mar 15, 2010 at 2:34 PM, KAMEZAWA Hiroyuki
> > <kamezawa.hiroyu@...fujitsu.com> wrote:
> > > On Mon, 15 Mar 2010 09:28:08 +0900
> > > Minchan Kim <minchan.kim@...il.com> wrote:

> > I think above scenario make error "use-after-free", again.
> > What prevent above scenario?
> > 
> I think this patch is not complete. 
> I guess this patch in [1/11] is trigger for the race.
> ==
> +
> +	/* Drop an anon_vma reference if we took one */
> +	if (anon_vma && atomic_dec_and_lock(&anon_vma->migrate_refcount, &anon_vma->lock)) {
> +		int empty = list_empty(&anon_vma->head);
> +		spin_unlock(&anon_vma->lock);
> +		if (empty)
> +			anon_vma_free(anon_vma);
> +	}
> ==
> If my understainding in above is correct, this "modify" freed anon_vma.
> Then, use-after-free happens. (In old implementation, there are no refcnt,
> so, there is no use-after-free ops.)
> 
Sorry, about above, my understanding was wrong. anon_vma->lock is modifed even
in old code. Sorry for noise.

-Kame








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