From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail137.messagelabs.com (mail137.messagelabs.com [216.82.249.19]) by kanga.kvack.org (Postfix) with ESMTP id 7FA546B0088 for ; Tue, 21 Dec 2010 05:49:30 -0500 (EST) Date: Tue, 21 Dec 2010 10:49:08 +0000 From: Mel Gorman Subject: Re: [PATCH] mm: migration: Use rcu_dereference_protected when dereferencing the radix tree slot during file page migration Message-ID: <20101221104907.GT13914@csn.ul.ie> References: <20101220152335.GR13914@csn.ul.ie> <20101220170146.GS13914@csn.ul.ie> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Sender: owner-linux-mm@kvack.org To: Minchan Kim Cc: Andrew Morton , gerald.schaefer@de.ibm.com, KAMEZAWA Hiroyuki , Milton Miller , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-ext4@vger.kernel.org, Ted Ts'o , Arun Bhanu , Heiko Carstens , Martin Schwidefsky List-ID: On Tue, Dec 21, 2010 at 08:48:50AM +0900, Minchan Kim wrote: > On Tue, Dec 21, 2010 at 2:01 AM, Mel Gorman wrote: > > On Mon, Dec 20, 2010 at 03:23:36PM +0000, Mel Gorman wrote: > >> migrate_pages() -> unmap_and_move() only calls rcu_read_lock() for anonymous > >> pages, as introduced by git commit 989f89c57e6361e7d16fbd9572b5da7d313b073d. > >> The point of the RCU protection there is part of getting a stable reference > >> to anon_vma and is only held for anon pages as file pages are locked > >> which is sufficient protection against freeing. > >> > >> However, while a file page's mapping is being migrated, the radix > >> tree is double checked to ensure it is the expected page. This uses > >> radix_tree_deref_slot() -> rcu_dereference() without the RCU lock held > >> triggering the following warning under CONFIG_PROVE_RCU. > >> > >> [ 173.674290] =================================================== > >> [ 173.676016] [ INFO: suspicious rcu_dereference_check() usage. ] > >> [ 173.676016] --------------------------------------------------- > >> [ 173.676016] include/linux/radix-tree.h:145 invoked rcu_dereference_check() without protection! > >> [ 173.676016] > >> [ 173.676016] other info that might help us debug this: > >> [ 173.676016] > >> [ 173.676016] > >> [ 173.676016] rcu_scheduler_active = 1, debug_locks = 0 > >> [ 173.676016] 1 lock held by hugeadm/2899: > >> [ 173.676016] #0: (&(&inode->i_data.tree_lock)->rlock){..-.-.}, at: [] migrate_page_move_mapping+0x40/0x1ab > >> [ 173.676016] > >> [ 173.676016] stack backtrace: > >> [ 173.676016] Pid: 2899, comm: hugeadm Not tainted 2.6.37-rc5-autobuild > >> [ 173.676016] Call Trace: > >> [ 173.676016] [] ? printk+0x14/0x1b > >> [ 173.676016] [] lockdep_rcu_dereference+0x7d/0x86 > >> [ 173.676016] [] migrate_page_move_mapping+0xca/0x1ab > >> [ 173.676016] [] migrate_page+0x23/0x39 > >> [ 173.676016] [] buffer_migrate_page+0x22/0x107 > >> [ 173.676016] [] ? buffer_migrate_page+0x0/0x107 > >> [ 173.676016] [] move_to_new_page+0x9a/0x1ae > >> [ 173.676016] [] migrate_pages+0x1e7/0x2fa > >> > >> This patch introduces radix_tree_deref_slot_protected() which calls > >> rcu_dereference_protected(). Users of it must pass in the mapping->tree_lock > >> that is protecting this dereference. Holding the tree lock protects against > >> parallel updaters of the radix tree meaning that rcu_dereference_protected > >> is allowable. > >> > >> Signed-off-by: Mel Gorman > >> --- > >> include/linux/radix-tree.h | 17 +++++++++++++++++ > >> mm/migrate.c | 4 ++-- > >> 2 files changed, 19 insertions(+), 2 deletions(-) > >> > >> diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h > >> index ab2baa5..a1f1672 100644 > >> --- a/include/linux/radix-tree.h > >> +++ b/include/linux/radix-tree.h > >> @@ -146,6 +146,23 @@ static inline void *radix_tree_deref_slot(void **pslot) > >> } > >> > >> /** > >> + * radix_tree_deref_slot_protected - dereference a slot without RCU lock but with tree lock held > >> + * @pslot: pointer to slot, returned by radix_tree_lookup_slot > >> + * Returns: item that was stored in that slot with any direct pointer flag > >> + * removed. > >> + * > >> + * Similar to radix_tree_deref_slot but only used during migration when a pages > >> + * mapping is being moved. The caller does not hold the RCU read lock but it > >> + * must hold the tree lock to prevent parallel updates. > >> + */ > >> +static inline void *radix_tree_deref_slot_protected(void **pslot, > >> + spinlock_t *treelock) > >> +{ > >> + BUG_ON(rcu_read_lock_held()); > > Hmm.. Why did you add the check? > If rcu_read_lock were already held, we wouldn't need this new API. > Because our earlier discussions assumed that RCU read lock was not held in this path. The check was added to ensure that assumption was correct, it wasn't. > > > > This was a bad idea. After some extended testing, it was obvious that > > this function can be called for swapcache pages with the RCU lock held. > > Paul, is it still permissible to use rcu_dereference_protected() or must > > I guess has no problem. > > > the RCU read lock not be held? > > > > > > -- > Kind regards, > Minchan Kim > -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom policy in Canada: sign http://dissolvethecrtc.ca/ Don't email: email@kvack.org