linux-kernel - Re: [BUG?] [Ext4] INFO: suspicious rcu_dereference

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Message-ID: <20101121153949.GD20947@barrios-desktop>
Date:	Mon, 22 Nov 2010 00:39:49 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Ted Ts'o <tytso@....edu>, linux-ext4@...r.kernel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Eric Sandeen <sandeen@...hat.com>
Subject: Re: [BUG?] [Ext4] INFO: suspicious rcu_dereference_check() usage

On Sun, Nov 21, 2010 at 08:30:24AM -0500, Ted Ts'o wrote:
> On Sun, Nov 21, 2010 at 07:26:11PM +0800, Arun Bhanu wrote:
> > I saw this in kernel log messages while testing 2.6.37-rc2. I think it
> > appeared while mounting an external hard-disk. I can't seem to
> > reproduce it.
> 
> I could be wrong but this looks like it's a bug in mm/migrate.c in
> migrate_page_move_mapping(): it is calling radix_tree_lookup_slot()
> without first taking an rcu_read_lock().
> 
> It was triggered by a memory allocation out of ext4_fill_super(),
> which then triggered a memory compaction/migration, but I don't
> believe it's otherwise related to the ext4 code.
> 
> Over to the linux-mm folks for confirmation...

I think it's no problem. 

That's because migration always holds lock_page on the file page.
So the page couldn't remove from radix. 


-- 
Kind regards,
Minchan Kim
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