Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755563Ab0LQQBb (ORCPT ); Fri, 17 Dec 2010 11:01:31 -0500 Received: from e8.ny.us.ibm.com ([32.97.182.138]:34426 "EHLO e8.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752139Ab0LQQB2 (ORCPT ); Fri, 17 Dec 2010 11:01:28 -0500 Date: Fri, 17 Dec 2010 08:01:03 -0800 From: "Paul E. McKenney" To: Mel Gorman Cc: Minchan Kim , gerald.schaefer@de.ibm.com, KAMEZAWA Hiroyuki , 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: <20101217160103.GC2181@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com 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-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20101217083912.GM13914@csn.ul.ie> User-Agent: Mutt/1.5.20 (2009-06-14) X-Content-Scanned: Fidelis XPS MAILER Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 10270 Lines: 247 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. > > > diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h > > index ab2baa5..135af1e 100644 > > --- a/include/linux/radix-tree.h > > +++ b/include/linux/radix-tree.h > > @@ -146,6 +146,20 @@ static inline void *radix_tree_deref_slot(void **pslot) > > } > > > > /** > > + * radix_tree_deref_slot_nocheck - dereference a slot without RCU check > > + * @pslot: pointer to slot, returned by radix_tree_lookup_slot > > + * Returns: item that was stored in that slot with any direct pointer flag > > + * removed. > > + * > > + * This functions works like radix_tree_deref_slot except it doesn't check > > + * RCU rule. Normally this funcion is used with update-side lock. > > + * You should use this function very carefully. > > + */ > > +static inline void *radix_tree_deref_slot_nocheck(void **pslot) > > +{ > > + return rcu_dereference_protected(*pslot, 1); > > +} > > For this, I had > > 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 > > In the documentation, I noted that the check might be without RCU but with > the knowledge that it's protected by the tree lock. I'm not a RCU expert > but this is only safe when you know there isn't a parallel updater and the > treelock should be preventing that, right? Yes, if you have prevented updates from happening, then it is OK to use rcu_dereference_protected() instead of rcu_dereference_check(). However, if RCU readers can invoke this code, things will break. By the way, this means that something like: rcu_dereference_protected(p, rcu_read_lock_held()) /* BUGGY!!! */ is just plain wrong. > Even so, other users of rcu_dereference_protected() check a lock condition > which I used tree lock for. I intended to read through the rest of > documentation properly this morning to determine if this was indeed the > right approach. > > I used the name _protected instead of _nocheck because the dereference > is still protected (by the tree lock) just not by RCU. Again, have to > check the documentation to ensure this is correct. Yep, this is indeed the case that rcu_dereference_protected() is for. > > +/** > > * 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 2eb2243..5be2841 100644 > > --- a/mm/migrate.c > > +++ b/mm/migrate.c > > @@ -244,7 +244,8 @@ static int migrate_page_move_mapping(struct > > address_space *mappin > > > > 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_nocheck(pslot) > > + != page) { > > spin_unlock_irq(&mapping->tree_lock); > > return -EAGAIN; > > } > > We only differed here by my passing in the &mapping->tree_lock Which should be optimized away during inlining, so no performance penalty. ;-) Thanx, Paul > > @@ -316,7 +317,8 @@ 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_nocheck(pslot) > > + != page) { > > spin_unlock_irq(&mapping->tree_lock); > > return -EAGAIN; > > } > > -- > Mel Gorman > Part-time Phd Student Linux Technology Center > University of Limerick IBM Dublin Software Lab -- 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/