From: Minchan Kim Subject: Re: [BUG?] memory hotplug: include/linux/radix-tree.h:145 invoked rcu_dereference_check() without protection! Date: Sat, 18 Dec 2010 00:13:54 +0900 Message-ID: <20101217151354.GB1609@barrios-desktop> 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-Transfer-Encoding: QUOTED-PRINTABLE 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 To: Mel Gorman Return-path: Received: from mail-pz0-f52.google.com ([209.85.210.52]:41161 "EHLO mail-pz0-f52.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753923Ab0LQPOJ (ORCPT ); Fri, 17 Dec 2010 10:14:09 -0500 Content-Disposition: inline In-Reply-To: <20101217083912.GM13914@csn.ul.ie> Sender: linux-ext4-owner@vger.kernel.org List-ID: 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_hugep= ages, see > > > below. Both cases are easily reproducible: memory unplug with big= page cache, > > > or adding large pages during run-time. > > > > > > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D > > > [ INFO: suspicious rcu_dereference_check() usage. ] > > > --------------------------------------------------- > > > include/linux/radix-tree.h:145 invoked rcu_dereference_check() wi= thout protection! > > > > > > other info that might help us debug this: > > > > > > > > > rcu_scheduler_active =3D 1, debug_locks =3D 0 > > > 1 lock held by bash/761: > > > =A0#0: =A0(&(&inode->i_data.tree_lock)->rlock){..-.-.}, at: [<000= 00000002263ae>] migrate_page_move_mapping+0x4a/0x2d8 > > > > > > stack backtrace: > > > CPU: 1 Not tainted 2.6.37-rc6 #4 > > > Process bash (pid: 761, task: 00000000181b5540, ksp: 00000000181b= b7f8) > > > 00000000181bb818 00000000181bb798 0000000000000002 00000000000000= 00 > > > =A0 =A0 =A0 00000000181bb838 00000000181bb7b0 00000000181bb7b0 00= 0000000056bafa > > > =A0 =A0 =A0 0000000000000000 000000003f42bdf0 0000000000000002 00= 0000001c43be30 > > > =A0 =A0 =A0 000003e00000000d 000003e00000000c 00000000181bb800 00= 00000000000000 > > > =A0 =A0 =A0 0000000000000000 0000000000100bfa 00000000181bb798 00= 000000181bb7d8 > > > Call Trace: > > > ([<0000000000100b02>] show_trace+0xee/0x144) > > > =A0[<000000000022654e>] migrate_page_move_mapping+0x1ea/0x2d8 > > > =A0[<0000000000226c80>] migrate_page+0x38/0x68 > > > =A0[<0000000000226d9a>] move_to_new_page+0xea/0x2bc > > > =A0[<000000000022785a>] migrate_pages+0x496/0x568 > > > =A0[<000000000021e24e>] compact_zone+0x432/0x7d8 > > > =A0[<000000000021e772>] compact_zone_order+0x9e/0xbc > > > =A0[<000000000021ed52>] try_to_compact_pages+0x1ba/0x24c > > > =A0[<00000000001e1afa>] __alloc_pages_nodemask+0x86a/0xa64 > > > =A0[<000000000021c80c>] alloc_fresh_huge_page.clone.2+0x68/0x18c > > > =A0[<000000000021cc4c>] set_max_huge_pages.clone.0+0xa4/0x1ac > > > =A0[<000000000021ce06>] hugetlb_sysctl_handler+0xb2/0xcc > > > =A0[<00000000002a6572>] proc_sys_call_handler+0xe6/0x10c > > > =A0[<00000000002a65be>] proc_sys_write+0x26/0x34 > > > =A0[<00000000002336e0>] vfs_write+0xac/0x18c > > > =A0[<00000000002338bc>] SyS_write+0x58/0xa8 > > > =A0[<0000000000113976>] sysc_noemu+0x16/0x1c > > > =A0[<0000020000162edc>] 0x20000162edc > > > INFO: lockdep is turned off. > > > > > > I honestly do not understand 100% why this is a false positive, s= eeing that > > > e.g. find_get_page() will also use radix_tree_deref_slot(), holdi= ng 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 affecte= d code paths, > > > even if it is not necessary for synchronization, would get rid of= the warning, > > > like in the following patch. Any ideas? > >=20 > > 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. > >=20 > > > > > > --- > > > =A0fs/hugetlbfs/inode.c | =A0 =A02 ++ > > > =A0mm/migrate.c =A0 =A0 =A0 =A0 | =A0 =A04 ++++ > > > =A02 files changed, 6 insertions(+) > > > > > > --- a/fs/hugetlbfs/inode.c > > > +++ b/fs/hugetlbfs/inode.c > > > @@ -580,7 +580,9 @@ static int hugetlbfs_migrate_page(struct > > > =A0{ > > > =A0 =A0 =A0 =A0int rc; > > > > > > + =A0 =A0 =A0 rcu_read_lock(); > > > =A0 =A0 =A0 =A0rc =3D migrate_huge_page_move_mapping(mapping, new= page, page); > > > + =A0 =A0 =A0 rcu_read_unlock(); > > > =A0 =A0 =A0 =A0if (rc) > > > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0return rc; > > > =A0 =A0 =A0 =A0migrate_page_copy(newpage, page); > > > --- a/mm/migrate.c > > > +++ b/mm/migrate.c > > > @@ -417,7 +417,9 @@ int migrate_page(struct address_space *m > > > > > > =A0 =A0 =A0 =A0BUG_ON(PageWriteback(page)); =A0 =A0/* Writeback m= ust be complete */ > > > > > > + =A0 =A0 =A0 rcu_read_lock(); > > > =A0 =A0 =A0 =A0rc =3D migrate_page_move_mapping(mapping, newpage,= page); > > > + =A0 =A0 =A0 rcu_read_unlock(); > > > > > > =A0 =A0 =A0 =A0if (rc) > > > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0return rc; > > > @@ -444,7 +446,9 @@ int buffer_migrate_page(struct address_s > > > > > > =A0 =A0 =A0 =A0head =3D page_buffers(page); > > > > > > + =A0 =A0 =A0 rcu_read_lock(); > > > =A0 =A0 =A0 =A0rc =3D migrate_page_move_mapping(mapping, newpage,= page); > > > + =A0 =A0 =A0 rcu_read_unlock(); > > > > > > =A0 =A0 =A0 =A0if (rc) > > > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0return rc; > > > > > > > > > > >=20 > >=20 > > How about this? > > Maybe Paul have better idea. > > (It's apparently be word-wrapped.) > >=20 >=20 > heh, I wrote a patch almost identical to this and ran it overnight fo= r 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 diffe= red Good.=20 > slightly in a number of respects though. >=20 > > 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) > > } > >=20 > > /** > > + * radix_tree_deref_slot_nocheck - dereference a slot withou= t RCU check > > + * @pslot: pointer to slot, returned by radix_tree_lookup_slot > > + * Returns: item that was stored in that slot with any direct p= ointer 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); > > +} >=20 > For this, I had >=20 > 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) > } > =20 > /** > + * 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 pointe= r flag > + * removed. > + * > + * For use with radix_tree_lookup_slot(). Caller must hold tree rea= d > + * 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 po= inter 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 >=20 > 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 exp= ert > but this is only safe when you know there isn't a parallel updater an= d the > treelock should be preventing that, right? I think so. >=20 > Even so, other users of rcu_dereference_protected() check a lock cond= ition > which I used tree lock for. I intended to read through the rest of > documentation properly this morning to determine if this was indeed t= he > right approach. >=20 > I used the name _protected instead of _nocheck because the dereferenc= e > is still protected (by the tree lock) just not by RCU. Again, have to > check the documentation to ensure this is correct. I agree _proected naming is good. >=20 > > +/** > > * 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 requ= ired > > 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 > >=20 > > expected_count =3D 2 + page_has_private(page); > > if (page_count(page) !=3D expected_count || > > - (struct page *)radix_tree_deref_slot(pslot)= !=3D page) { > > + (struct page *)radix_tree_deref_slot_nochec= k(pslot) > > + !=3D page) { > > spin_unlock_irq(&mapping->tree_lock); > > return -EAGAIN; > > } >=20 > We only differed here by my passing in the &mapping->tree_lock I will add the comment in your formal patch. >=20 >=20 >=20 > > @@ -316,7 +317,8 @@ int migrate_huge_page_move_mapping(struct > > address_space *mapping, > >=20 > > expected_count =3D 2 + page_has_private(page); > > if (page_count(page) !=3D expected_count || > > - (struct page *)radix_tree_deref_slot(pslot) !=3D page) = { > > + (struct page *)radix_tree_deref_slot_nocheck(pslot) > > + !=3D page) { > > spin_unlock_irq(&mapping->tree_lock); > > return -EAGAIN; > > } >=20 > --=20 > Mel Gorman > Part-time Phd Student Linux Technology Cente= r > University of Limerick IBM Dublin Software La= b --=20 Kind regards, Minchan Kim -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html