From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail202.messagelabs.com (mail202.messagelabs.com [216.82.254.227]) by kanga.kvack.org (Postfix) with ESMTP id 05A976B0022 for ; Sun, 8 May 2011 15:41:43 -0400 (EDT) Received: from hpaq1.eem.corp.google.com (hpaq1.eem.corp.google.com [172.25.149.1]) by smtp-out.google.com with ESMTP id p48Jfe4i011422 for ; Sun, 8 May 2011 12:41:40 -0700 Received: from pxi7 (pxi7.prod.google.com [10.243.27.7]) by hpaq1.eem.corp.google.com with ESMTP id p48Jfb1f028429 (version=TLSv1/SSLv3 cipher=RC4-SHA bits=128 verify=NOT) for ; Sun, 8 May 2011 12:41:39 -0700 Received: by pxi7 with SMTP id 7so3196263pxi.30 for ; Sun, 08 May 2011 12:41:37 -0700 (PDT) Date: Sun, 8 May 2011 12:41:48 -0700 (PDT) From: Hugh Dickins Subject: [PATCH 1/3] tmpfs: fix race between umount and writepage In-Reply-To: <4DC691D0.6050104@parallels.com> Message-ID: References: <4DAFD0B1.9090603@parallels.com> <20110421064150.6431.84511.stgit@localhost6> <20110421124424.0a10ed0c.akpm@linux-foundation.org> <4DB0FE8F.9070407@parallels.com> <4DC4D9A6.9070103@parallels.com> <4DC691D0.6050104@parallels.com> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton Cc: Konstantin Khlebnikov , linux-mm@kvack.org, linux-kernel@vger.kernel.org Konstanin Khlebnikov reports that a dangerous race between umount and shmem_writepage can be reproduced by this script: for i in {1..300} ; do mkdir $i while true ; do mount -t tmpfs none $i dd if=/dev/zero of=$i/test bs=1M count=$(($RANDOM % 100)) umount $i done & done on a 6xCPU node with 8Gb RAM: kernel very unstable after this accident. =) Kernel log: VFS: Busy inodes after unmount of tmpfs. Self-destruct in 5 seconds. Have a nice day... WARNING: at lib/list_debug.c:53 __list_del_entry+0x8d/0x98() list_del corruption. prev->next should be ffff880222fdaac8, but was (null) Pid: 11222, comm: mount.tmpfs Not tainted 2.6.39-rc2+ #4 Call Trace: warn_slowpath_common+0x80/0x98 warn_slowpath_fmt+0x41/0x43 __list_del_entry+0x8d/0x98 evict+0x50/0x113 iput+0x138/0x141 ... BUG: unable to handle kernel paging request at ffffffffffffffff IP: shmem_free_blocks+0x18/0x4c Pid: 10422, comm: dd Tainted: G W 2.6.39-rc2+ #4 Call Trace: shmem_recalc_inode+0x61/0x66 shmem_writepage+0xba/0x1dc pageout+0x13c/0x24c shrink_page_list+0x28e/0x4be shrink_inactive_list+0x21f/0x382 ... shmem_writepage() calls igrab() on the inode for the page which came from page reclaim, to add it later into shmem_swaplist for swapoff operation. This igrab() can race with super-block deactivating process: shrink_inactive_list() deactivate_super() pageout() tmpfs_fs_type->kill_sb() shmem_writepage() kill_litter_super() generic_shutdown_super() evict_inodes() igrab() atomic_read(&inode->i_count) skip-inode iput() if (!list_empty(&sb->s_inodes)) printk("VFS: Busy inodes after... This igrap-iput pair was added in commit 1b1b32f2c6f6 "tmpfs: fix shmem_swaplist races" based on incorrect assumptions: igrab() protects the inode from concurrent eviction by deletion, but it does nothing to protect it from concurrent unmounting, which goes ahead despite the raised i_count. So this use of igrab() was wrong all along, but the race made much worse in 2.6.37 when commit 63997e98a3be "split invalidate_inodes()" replaced two attempts at invalidate_inodes() by a single evict_inodes(). Konstantin posted a plausible patch, raising sb->s_active too: I'm unsure whether it was correct or not; but burnt once by igrab(), I am sure that we don't want to rely more deeply upon externals here. Fix it by adding the inode to shmem_swaplist earlier, while the page lock on page in page cache still secures the inode against eviction, without artifically raising i_count. It was originally added later because shmem_unuse_inode() is liable to remove an inode from the list while it's unswapped; but we can guard against that by taking spinlock before dropping mutex. Reported-by: Konstantin Khlebnikov Signed-off-by: Hugh Dickins Tested-by: Konstantin Khlebnikov Cc: stable@kernel.org --- mm/shmem.c | 31 ++++++++++++++++++++----------- 1 file changed, 20 insertions(+), 11 deletions(-) --- 2.6.39-rc6/mm/shmem.c 2011-04-28 09:52:49.066135001 -0700 +++ tmpfs1/mm/shmem.c 2011-05-07 17:38:00.648660817 -0700 @@ -1039,6 +1039,7 @@ static int shmem_writepage(struct page * struct address_space *mapping; unsigned long index; struct inode *inode; + bool unlock_mutex = false; BUG_ON(!PageLocked(page)); mapping = page->mapping; @@ -1064,7 +1065,26 @@ static int shmem_writepage(struct page * else swap.val = 0; + /* + * Add inode to shmem_unuse()'s list of swapped-out inodes, + * if it's not already there. Do it now because we cannot take + * mutex while holding spinlock, and must do so before the page + * is moved to swap cache, when its pagelock no longer protects + * the inode from eviction. But don't unlock the mutex until + * we've taken the spinlock, because shmem_unuse_inode() will + * prune a !swapped inode from the swaplist under both locks. + */ + if (swap.val && list_empty(&info->swaplist)) { + mutex_lock(&shmem_swaplist_mutex); + /* move instead of add in case we're racing */ + list_move_tail(&info->swaplist, &shmem_swaplist); + unlock_mutex = true; + } + spin_lock(&info->lock); + if (unlock_mutex) + mutex_unlock(&shmem_swaplist_mutex); + if (index >= info->next_index) { BUG_ON(!(info->flags & SHMEM_TRUNCATE)); goto unlock; @@ -1084,21 +1104,10 @@ static int shmem_writepage(struct page * delete_from_page_cache(page); shmem_swp_set(info, entry, swap.val); shmem_swp_unmap(entry); - if (list_empty(&info->swaplist)) - inode = igrab(inode); - else - inode = NULL; spin_unlock(&info->lock); swap_shmem_alloc(swap); BUG_ON(page_mapped(page)); swap_writepage(page, wbc); - if (inode) { - mutex_lock(&shmem_swaplist_mutex); - /* move instead of add in case we're racing */ - list_move_tail(&info->swaplist, &shmem_swaplist); - mutex_unlock(&shmem_swaplist_mutex); - iput(inode); - } return 0; } -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. 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