Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752259Ab0LQJ2w (ORCPT ); Fri, 17 Dec 2010 04:28:52 -0500 Received: from gir.skynet.ie ([193.1.99.77]:41453 "EHLO gir.skynet.ie" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751250Ab0LQJ2s (ORCPT ); Fri, 17 Dec 2010 04:28:48 -0500 Date: Fri, 17 Dec 2010 09:28:28 +0000 From: Mel Gorman To: Minchan Kim Cc: gerald.schaefer@de.ibm.com, KAMEZAWA Hiroyuki , paulmck@linux.vnet.ibm.com, Milton Miller , linux-kernel@vger.kernel.org, linux-mm@vger.kernel.org, linux-ext4@vger.kernel.org, "Ted Ts'o" , Arun Bhanu , Andrew Morton , Heiko Carstens , Martin Schwidefsky Subject: Re: [BUG?] memory hotplug: include/linux/radix-tree.h:145 invoked rcu_dereference_check() without protection! Message-ID: <20101217092828.GN13914@csn.ul.ie> References: <20101121153949.GD20947@barrios-desktop> <20101121173726.GG23423@thunk.org> <20101122061619.GA2764@linux.vnet.ibm.com> <1291748509.19276.62.camel@thinkpad> <20101208101947.b0646226.kamezawa.hiroyu@jp.fujitsu.com> <1292507421.4885.19.camel@thinkpad> <20101217083912.GM13914@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: <20101217083912.GM13914@csn.ul.ie> User-Agent: Mutt/1.5.17+20080114 (2008-01-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 10199 Lines: 232 On Fri, Dec 17, 2010 at 08:39:12AM +0000, Mel Gorman wrote: > On Fri, Dec 17, 2010 at 09:04:13AM +0900, Minchan Kim wrote: > > On Thu, Dec 16, 2010 at 10:50 PM, Gerald Schaefer > > wrote: > > > I got the same warning now after increasing /proc/sys/vm/nr_hugepages, see > > > below. Both cases are easily reproducible: memory unplug with big page cache, > > > or adding large pages during run-time. > > > > > > =================================================== > > > [ INFO: suspicious rcu_dereference_check() usage. ] > > > --------------------------------------------------- > > > include/linux/radix-tree.h:145 invoked rcu_dereference_check() without protection! > > > > > > other info that might help us debug this: > > > > > > > > > rcu_scheduler_active = 1, debug_locks = 0 > > > 1 lock held by bash/761: > > > ?#0: ?(&(&inode->i_data.tree_lock)->rlock){..-.-.}, at: [<00000000002263ae>] migrate_page_move_mapping+0x4a/0x2d8 > > > > > > stack backtrace: > > > CPU: 1 Not tainted 2.6.37-rc6 #4 > > > Process bash (pid: 761, task: 00000000181b5540, ksp: 00000000181bb7f8) > > > 00000000181bb818 00000000181bb798 0000000000000002 0000000000000000 > > > ? ? ? 00000000181bb838 00000000181bb7b0 00000000181bb7b0 000000000056bafa > > > ? ? ? 0000000000000000 000000003f42bdf0 0000000000000002 000000001c43be30 > > > ? ? ? 000003e00000000d 000003e00000000c 00000000181bb800 0000000000000000 > > > ? ? ? 0000000000000000 0000000000100bfa 00000000181bb798 00000000181bb7d8 > > > Call Trace: > > > ([<0000000000100b02>] show_trace+0xee/0x144) > > > ?[<000000000022654e>] migrate_page_move_mapping+0x1ea/0x2d8 > > > ?[<0000000000226c80>] migrate_page+0x38/0x68 > > > ?[<0000000000226d9a>] move_to_new_page+0xea/0x2bc > > > ?[<000000000022785a>] migrate_pages+0x496/0x568 > > > ?[<000000000021e24e>] compact_zone+0x432/0x7d8 > > > ?[<000000000021e772>] compact_zone_order+0x9e/0xbc > > > ?[<000000000021ed52>] try_to_compact_pages+0x1ba/0x24c > > > ?[<00000000001e1afa>] __alloc_pages_nodemask+0x86a/0xa64 > > > ?[<000000000021c80c>] alloc_fresh_huge_page.clone.2+0x68/0x18c > > > ?[<000000000021cc4c>] set_max_huge_pages.clone.0+0xa4/0x1ac > > > ?[<000000000021ce06>] hugetlb_sysctl_handler+0xb2/0xcc > > > ?[<00000000002a6572>] proc_sys_call_handler+0xe6/0x10c > > > ?[<00000000002a65be>] proc_sys_write+0x26/0x34 > > > ?[<00000000002336e0>] vfs_write+0xac/0x18c > > > ?[<00000000002338bc>] SyS_write+0x58/0xa8 > > > ?[<0000000000113976>] sysc_noemu+0x16/0x1c > > > ?[<0000020000162edc>] 0x20000162edc > > > INFO: lockdep is turned off. > > > > > > I honestly do not understand 100% why this is a false positive, seeing that > > > e.g. find_get_page() will also use radix_tree_deref_slot(), holding only the > > > rcu_read_lock, while migrate_page_move_mapping() has no rcu_read_lock() but > > > the &mapping->tree_lock instead. So I'm not quite sure how to fix this > > > properly, but simply adding rcu_read_lock/unlock() to the affected code paths, > > > even if it is not necessary for synchronization, would get rid of the warning, > > > like in the following patch. Any ideas? > > > > In case of anon page, we hold rcu_read_lock in unmap_and_move. > > The problem is file-backed page. In case of that, we hold lock_page > > and mapping->tree_lock as update-side lock. > > So we don't need rcu_read_lock. > > > > > > > > --- > > > ?fs/hugetlbfs/inode.c | ? ?2 ++ > > > ?mm/migrate.c ? ? ? ? | ? ?4 ++++ > > > ?2 files changed, 6 insertions(+) > > > > > > --- a/fs/hugetlbfs/inode.c > > > +++ b/fs/hugetlbfs/inode.c > > > @@ -580,7 +580,9 @@ static int hugetlbfs_migrate_page(struct > > > ?{ > > > ? ? ? ?int rc; > > > > > > + ? ? ? rcu_read_lock(); > > > ? ? ? ?rc = migrate_huge_page_move_mapping(mapping, newpage, page); > > > + ? ? ? rcu_read_unlock(); > > > ? ? ? ?if (rc) > > > ? ? ? ? ? ? ? ?return rc; > > > ? ? ? ?migrate_page_copy(newpage, page); > > > --- a/mm/migrate.c > > > +++ b/mm/migrate.c > > > @@ -417,7 +417,9 @@ int migrate_page(struct address_space *m > > > > > > ? ? ? ?BUG_ON(PageWriteback(page)); ? ?/* Writeback must be complete */ > > > > > > + ? ? ? rcu_read_lock(); > > > ? ? ? ?rc = migrate_page_move_mapping(mapping, newpage, page); > > > + ? ? ? rcu_read_unlock(); > > > > > > ? ? ? ?if (rc) > > > ? ? ? ? ? ? ? ?return rc; > > > @@ -444,7 +446,9 @@ int buffer_migrate_page(struct address_s > > > > > > ? ? ? ?head = page_buffers(page); > > > > > > + ? ? ? rcu_read_lock(); > > > ? ? ? ?rc = migrate_page_move_mapping(mapping, newpage, page); > > > + ? ? ? rcu_read_unlock(); > > > > > > ? ? ? ?if (rc) > > > ? ? ? ? ? ? ? ?return rc; > > > > > > > > > > > > > > > How about this? > > Maybe Paul have better idea. > > (It's apparently be word-wrapped.) > > > > heh, I wrote a patch almost identical to this and ran it overnight for testing > (test was a memory consumer running while a parallel process grew and shrunk > the hugepage pool). It passes but that is hardly a surprise. We differed > slightly in a number of respects though. > For completeness, this is what I tested last night. There are two "confirms" in the changelog that I intended to work out today but maybe someone can confirm faster. ==== CUT HERE ==== mm: migration: Use rcu_dereference_protected when dereferencing the radix tree slot during file page migration 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. [ 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. Based on the locking hierarchy described in mm/filemap.c, holding the tree lock is protecting the radix tree from concurrent updaters in all cases (Confirm that no case has been missed). According to Documentation/RCU/lockdep.txt, if there is a guarantee that no parallel updaters exist, use of rcu_dereference_protected() is allowed (Confirm this is accurate?). Signed-off-by: Mel Gorman --- include/linux/radix-tree.h | 19 +++++++++++++++++++ mm/migrate.c | 4 ++-- 2 files changed, 21 insertions(+), 2 deletions(-) diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h index ab2baa5..252d21c 100644 --- a/include/linux/radix-tree.h +++ b/include/linux/radix-tree.h @@ -146,6 +146,25 @@ 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. + * + * For use with radix_tree_lookup_slot(). Caller must hold tree read + * locked across slot lookup and dereference. Not required if write lock is + * held (ie. items cannot be concurrently inserted). + * + * radix_tree_deref_retry must be used to confirm validity of the pointer if + * only the read lock is held. + */ +static inline void *radix_tree_deref_slot_protected(void **pslot, + spinlock_t *treelock) +{ + return rcu_dereference_protected(*pslot, lockdep_is_held(treelock)); +} + +/** * radix_tree_deref_retry - check radix_tree_deref_slot * @arg: pointer returned by radix_tree_deref_slot * Returns: 0 if retry is not required, otherwise retry is required diff --git a/mm/migrate.c b/mm/migrate.c index fe5a3c6..7d4686a 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -244,7 +244,7 @@ static int migrate_page_move_mapping(struct address_space *mapping, expected_count = 2 + page_has_private(page); if (page_count(page) != expected_count || - (struct page *)radix_tree_deref_slot(pslot) != page) { + (struct page *)radix_tree_deref_slot_protected(pslot, &mapping->tree_lock) != page) { spin_unlock_irq(&mapping->tree_lock); return -EAGAIN; } @@ -316,7 +316,7 @@ int migrate_huge_page_move_mapping(struct address_space *mapping, expected_count = 2 + page_has_private(page); if (page_count(page) != expected_count || - (struct page *)radix_tree_deref_slot(pslot) != page) { + (struct page *)radix_tree_deref_slot_protected(pslot, &mapping->tree_lock) != page) { spin_unlock_irq(&mapping->tree_lock); return -EAGAIN; } -- 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/