linux-kernel - Re: [PATCH] mm: migration: Use rcu_dereference_protected when dereferencing the radix tree slot during file page migration

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Message-ID: <AANLkTiniXU9B5YpQ+hknOSF-mPig2z9UqqBWz-JwQjDL@mail.gmail.com>
Date:	Tue, 21 Dec 2010 08:48:50 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Mel Gorman <mel@....ul.ie>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	gerald.schaefer@...ibm.com,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Milton Miller <miltonm@....com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, linux-ext4@...r.kernel.org,
	"Ted Ts'o" <tytso@....edu>, Arun Bhanu <ab@...nbhanu.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH] mm: migration: Use rcu_dereference_protected when
 dereferencing the radix tree slot during file page migration

On Tue, Dec 21, 2010 at 2:01 AM, Mel Gorman <mel@....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 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: [<c10e3d2b>] 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]  [<c128cc01>] ? printk+0x14/0x1b
>> [  173.676016]  [<c1063502>] lockdep_rcu_dereference+0x7d/0x86
>> [  173.676016]  [<c10e3db5>] migrate_page_move_mapping+0xca/0x1ab
>> [  173.676016]  [<c10e41ad>] migrate_page+0x23/0x39
>> [  173.676016]  [<c10e491b>] buffer_migrate_page+0x22/0x107
>> [  173.676016]  [<c10e48f9>] ? buffer_migrate_page+0x0/0x107
>> [  173.676016]  [<c10e425d>] move_to_new_page+0x9a/0x1ae
>> [  173.676016]  [<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->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 <mel@....ul.ie>
>> ---
>>  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
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