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

From: Theodore Tso
Date: Sat Sep 05 2009 - 08:59:52 EST

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@xxxxxxxxxxx>

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

> 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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