From: Theodore Tso Subject: Re: [PATCH] ext2: fix unbalanced kmap()/kunmap() Date: Sat, 5 Sep 2009 08:59:30 -0400 Message-ID: <20090905125930.GF16217@mit.edu> References: Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Linus Torvalds , linux-ext4@vger.kernel.org, lkml To: Nicolas Pitre Return-path: Received: from THUNK.ORG ([69.25.196.29]:50911 "EHLO thunker.thunk.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751053AbZIEM7n (ORCPT ); Sat, 5 Sep 2009 08:59:43 -0400 Content-Disposition: inline In-Reply-To: Sender: linux-ext4-owner@vger.kernel.org List-ID: 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 Acked-by: "Theodore Ts'o" > 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