From: Minchan Kim <minchan.kim@gmail.com>
Subject: Re: [PATCH] mm: migration: Use rcu_dereference_protected when
 dereferencing the radix tree slot during file page migration
Date: Tue, 21 Dec 2010 08:48:50 +0900
Message-ID: <AANLkTiniXU9B5YpQ+hknOSF-mPig2z9UqqBWz-JwQjDL@mail.gmail.com>
References: <20101220152335.GR13914@csn.ul.ie>
	<20101220170146.GS13914@csn.ul.ie>
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Cc: Andrew Morton <akpm@linux-foundation.org>,
	gerald.schaefer@de.ibm.com,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
	Milton Miller <miltonm@bga.com>, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org, linux-ext4@vger.kernel.org,
	"Ted Ts'o" <tytso@mit.edu>, Arun Bhanu <ab@arunbhanu.com>,
	Heiko Carstens <heiko.carstens@de.ibm.com>,
	Martin Schwidefsky <schwidefsky@de.ibm.com>
To: Mel Gorman <mel@csn.ul.ie>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <20101220170146.GS13914@csn.ul.ie>
Sender: linux-kernel-owner@vger.kernel.org
List-Id: linux-ext4.vger.kernel.org

On Tue, Dec 21, 2010 at 2:01 AM, Mel Gorman <mel@csn.ul.ie> 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 a=
nonymous
>> pages, as introduced by git commit 989f89c57e6361e7d16fbd9572b5da7d3=
13b073d.
>> The point of the RCU protection there is part of getting a stable re=
ference
>> 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 he=
ld
>> triggering the following warning under CONFIG_PROVE_RCU.
>>
>> [ =A0173.674290] =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
>> [ =A0173.676016] [ INFO: suspicious rcu_dereference_check() usage. ]
>> [ =A0173.676016] ---------------------------------------------------
>> [ =A0173.676016] include/linux/radix-tree.h:145 invoked rcu_derefere=
nce_check() without protection!
>> [ =A0173.676016]
>> [ =A0173.676016] other info that might help us debug this:
>> [ =A0173.676016]
>> [ =A0173.676016]
>> [ =A0173.676016] rcu_scheduler_active =3D 1, debug_locks =3D 0
>> [ =A0173.676016] 1 lock held by hugeadm/2899:
>> [ =A0173.676016] =A0#0: =A0(&(&inode->i_data.tree_lock)->rlock){..-.=
-.}, at: [<c10e3d2b>] migrate_page_move_mapping+0x40/0x1ab
>> [ =A0173.676016]
>> [ =A0173.676016] stack backtrace:
>> [ =A0173.676016] Pid: 2899, comm: hugeadm Not tainted 2.6.37-rc5-aut=
obuild
>> [ =A0173.676016] Call Trace:
>> [ =A0173.676016] =A0[<c128cc01>] ? printk+0x14/0x1b
>> [ =A0173.676016] =A0[<c1063502>] lockdep_rcu_dereference+0x7d/0x86
>> [ =A0173.676016] =A0[<c10e3db5>] migrate_page_move_mapping+0xca/0x1a=
b
>> [ =A0173.676016] =A0[<c10e41ad>] migrate_page+0x23/0x39
>> [ =A0173.676016] =A0[<c10e491b>] buffer_migrate_page+0x22/0x107
>> [ =A0173.676016] =A0[<c10e48f9>] ? buffer_migrate_page+0x0/0x107
>> [ =A0173.676016] =A0[<c10e425d>] move_to_new_page+0x9a/0x1ae
>> [ =A0173.676016] =A0[<c10e47e6>] 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->t=
ree_lock
>> that is protecting this dereference. Holding the tree lock protects =
against
>> parallel updaters of the radix tree meaning that rcu_dereference_pro=
tected
>> is allowable.
>>
>> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
>> ---
>> =A0include/linux/radix-tree.h | =A0 17 +++++++++++++++++
>> =A0mm/migrate.c =A0 =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A04 ++--
>> =A02 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)
>> =A0}
>>
>> =A0/**
>> + * radix_tree_deref_slot_protected =A0 - dereference a slot without=
 RCU lock but with tree lock held
>> + * @pslot: =A0 pointer to slot, returned by radix_tree_lookup_slot
>> + * Returns: =A0item that was stored in that slot with any direct po=
inter flag
>> + * =A0 =A0 =A0 =A0 =A0 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 lo=
ck but it
>> + * must hold the tree lock to prevent parallel updates.
>> + */
>> +static inline void *radix_tree_deref_slot_protected(void **pslot,
>> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0=
 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 spinlock_t *treelock)
>> +{
>> + =A0 =A0 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 hel=
d.
> Paul, is it still permissible to use rcu_dereference_protected() or m=
ust

I guess has no problem.

> the RCU read lock not be held?
>



--=20
Kind regards,
Minchan Kim