linux-kernel - Re: Ugly rmap NULL ptr deref oopsie on hibernate (was Linux 2.6.34-rc3)

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Date:	Wed, 07 Apr 2010 01:45:27 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Borislav Petkov <bp@...en8.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Nick Piggin <npiggin@...e.de>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>
Subject: Re: Ugly rmap NULL ptr deref oopsie on hibernate (was Linux
 2.6.34-rc3)

On Tue, 2010-04-06 at 09:28 -0700, Linus Torvalds wrote:
> 
> On Wed, 7 Apr 2010, Minchan Kim wrote:
> > > 
> > > However, that made me look at the PAGE_MIGRATION case. That seems to be 
> > > just broken. It's doing that page_anon_vma() + spin_lock without holding 
> > > any RCU locks, so there is no guarantee that anon_vma there is at all 
> > > valid.
> > 
> > FYI, recently there is a patch about migration case. 
> > http://lkml.org/lkml/2010/4/2/145
> 
> No, I'm talking about rmap_walk_anon():
> 
>         anon_vma = page_anon_vma(page);
>         if (!anon_vma)
>                 return ret;
>         spin_lock(&anon_vma->lock);
> 
> which seems to be simply buggy. The anon_vma may not exist any more, 
> because an RCU event might have really freed the page between looking it 
> up and locking it.
> 
> 			Linus

unmap_and_move
	remove_migration_ptes
		rmap_walk
			rmap_walk_anon

We always has rcu_read_lock about anon page in unmap_and_move.
So I think it's not buggy. What am I missing?


-- 
Kind regards,
Minchan Kim


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