linux-ext4 - Re: [PATCH] ext2: fix unbalanced kmap()/kunmap()

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Message-ID: <20090905125930.GF16217@mit.edu>
Date:	Sat, 5 Sep 2009 08:59:30 -0400
From:	Theodore Tso <tytso@....edu>
To:	Nicolas Pitre <nico@....org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-ext4@...r.kernel.org, lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ext2: fix unbalanced kmap()/kunmap()

On Sat, Sep 05, 2009 at 12:25:37AM -0400, Nicolas Pitre wrote:
> In ext2_rename(), dir_page is acquired through ext2_dotdot().  It is 
> then released through ext2_set_link() but only if old_dir != new_dir. 
> Failing that, the pkmap reference count is never decremented and the 
> page remains pinned forever.  Repeat that a couple times with highmem 
> pages and all pkmap slots get exhausted, and every further kmap() calls 
> end up stalling on the pkmap_map_wait queue at which point the whole 
> system comes to a halt.
> 
> Signed-off-by: Nicolas Pitre <nico@...vell.com>

Acked-by: "Theodore Ts'o" <tytso@....edu>

> I ran into this issue while testing highmem on ARM by running the git 
> test suite in a loop with 3 parallel instances.  Using the right mv 
> sequence in a script would constitute a pretty simple recipe for a 
> local DoS on systems running ext2 and highmem.  No idea if ext3 or ext4 
> have the same issue.

This is an ext2-only issue; for journalling reasons ext3 and ext4 read
and write the directory via buffer heads.  So this issue won't apply
for ext3 or ext4.

						- Ted
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