Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933326Ab0LTXsw (ORCPT ); Mon, 20 Dec 2010 18:48:52 -0500 Received: from mail-iw0-f174.google.com ([209.85.214.174]:50043 "EHLO mail-iw0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933143Ab0LTXsv convert rfc822-to-8bit (ORCPT ); Mon, 20 Dec 2010 18:48:51 -0500 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :cc:content-type:content-transfer-encoding; b=Pn0f7DpbrF/JoPusdbtJrH4mmkqs0d2N31Dm97n8HbzhuHsEJLaB47vNmv79j0KAeh pYo9290lW4uyClHV1I4tXuQdJ9KH176F0XdRSpXl6xsrxZK6bgAy5ksoTas5rREgLyZp z1fEnP3Gy1Q5prEN+jbriCZij/ccClZv10m5Q= MIME-Version: 1.0 In-Reply-To: <20101220170146.GS13914@csn.ul.ie> References: <20101220152335.GR13914@csn.ul.ie> <20101220170146.GS13914@csn.ul.ie> Date: Tue, 21 Dec 2010 08:48:50 +0900 Message-ID: Subject: Re: [PATCH] mm: migration: Use rcu_dereference_protected when dereferencing the radix tree slot during file page migration From: Minchan Kim To: Mel Gorman 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 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4261 Lines: 94 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. > > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/