From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756566Ab1E1UOV (ORCPT ); Sat, 28 May 2011 16:14:21 -0400 Received: from smtp-out.google.com ([216.239.44.51]:49846 "EHLO smtp-out.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756193Ab1E1UOT (ORCPT ); Sat, 28 May 2011 16:14:19 -0400 DomainKey-Signature: a=rsa-sha1; c=nofws; d=google.com; s=beta; h=date:from:x-x-sender:to:cc:subject:message-id:user-agent :mime-version:content-type; b=bFgqFJ9TIETdkeez9GLbijFS40YkMmvd8UG3249D+FmyL1PSWcpqFgkeYR/BVkdVav VBrPPBItgBQ++12Hg/1w== Date: Sat, 28 May 2011 13:14:09 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@sister.anvils To: Linus Torvalds cc: Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: [PATCH] tmpfs: fix race between truncate and writepage Message-ID: User-Agent: Alpine 2.00 (LSU 1167 2008-08-23) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-System-Of-Record: true Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org While running fsx on tmpfs with a memhog then swapoff, swapoff was hanging (interruptibly), repeatedly failing to locate the owner of a 0xff entry in the swap_map. Although shmem_writepage() does abandon when it sees incoming page index is beyond eof, there was still a window in which shmem_truncate_range() could come in between writepage's dropping lock and updating swap_map, find the half-completed swap_map entry, and in trying to free it, leave it in a state that swap_shmem_alloc() could not correct. Arguably a bug in __swap_duplicate()'s and swap_entry_free()'s handling of the different cases, but easiest to fix by moving swap_shmem_alloc() under cover of the lock. More interesting than the bug: it's been there since 2.6.33, why could I not see it with earlier kernels? The mmotm of two weeks ago seems to have some magic for generating races, this is just one of three I found. With yesterday's git I first saw this in mainline, bisected in search of that magic, but the easy reproducibility evaporated. Oh well, fix the bug. Signed-off-by: Hugh Dickins Cc: stable@kernel.org --- mm/shmem.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) --- linux.orig/mm/shmem.c 2011-05-27 19:05:27.000000000 -0700 +++ linux/mm/shmem.c 2011-05-27 19:45:44.194813695 -0700 @@ -1114,8 +1114,8 @@ static int shmem_writepage(struct page * delete_from_page_cache(page); shmem_swp_set(info, entry, swap.val); shmem_swp_unmap(entry); - spin_unlock(&info->lock); swap_shmem_alloc(swap); + spin_unlock(&info->lock); BUG_ON(page_mapped(page)); swap_writepage(page, wbc); return 0;