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

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

[Mel Gorman]

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@csn.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());
+	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;
 	}

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

[Mel Gorman]

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@csn.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());

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
the RCU read lock not be held?

> +	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;
>  	}
> 

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

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

[Minchan Kim]

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 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@csn.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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Re: [PATCH] mm: migration: Use rcu_dereference_protected when dereferencing the radix tree slot during file page migration

[Mel Gorman]

On Tue, Dec 21, 2010 at 08:48:50AM +0900, Minchan Kim wrote:
> 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 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@csn.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.
> 

Because our earlier discussions assumed that RCU read lock was not held
in this path. The check was added to ensure that assumption was correct,
it wasn't.

> >
> > 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
> 

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

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

[Milton Miller]

[ Add Paul back to the CC list, and also Dipankar.
 Hopefully I killed the mime encodings correctly ]

On Tue, 21 Dec 2010 at 08:48:50 +0900, Minchan Kim wrote:
> 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 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@csn.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 RCUlock 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.

I'm not Paul but I can read the code in include/linux/rcuupdate.h.

Holding rcu_read_lock_held isn't a problem, but using protected with
just the read lock is.


> 
> >
> > 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.

No this is a problem

.. because __rcu_dereference_protected doesn't include the 
smp_read_barrier_depends() that is needed in the rcu only reference path.


Either we need two helpers, one for when the tree is write locked and
one when the tree is only rcu read locked, or we use __rcu_dereference_check
with check = 1.

> 
> > the RCU read lock not be held?
> >
> 
> Minchan Kim

milton

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

[Mel Gorman]

On Tue, Dec 21, 2010 at 01:16:23AM -0600, Milton Miller wrote:
> 
> [ Add Paul back to the CC list, and also Dipankar.
>  Hopefully I killed the mime encodings correctly ]
> 
> On Tue, 21 Dec 2010 at 08:48:50 +0900, Minchan Kim wrote:
> > 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 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@csn.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 RCUlock 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.
> 
> I'm not Paul but I can read the code in include/linux/rcuupdate.h.
> 
> Holding rcu_read_lock_held isn't a problem, but using protected with
> just the read lock is.
> 

Bah, this was extremely careless of me as it's even written in teh
documentation. In this specific case, it's simply allowed to ignore whether
the RCU read lock is held or not and the BUG_ON check was unnecessary. The
tree lock protects against parallel updaters which is what we really care
about for using _protected.

In a later cycle, I should look at reducing the RCU read lock hold time
in migration. The main thing it's protecting is getting a stable
reference to anon_vma and it's held longer than is necessary for that.
In the meantime, can anyone spot a problem with this patch?

==== 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: [<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@csn.ul.ie>
---
 include/linux/radix-tree.h |   16 ++++++++++++++++
 mm/migrate.c               |    4 ++--
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h
index ab2baa5..23241c2 100644
--- a/include/linux/radix-tree.h
+++ b/include/linux/radix-tree.h
@@ -146,6 +146,22 @@ 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)
+{
+	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;
 	}

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

[Minchan Kim]

On Tue, Dec 21, 2010 at 9:26 PM, Mel Gorman <mel@csn.ul.ie> wrote:
> On Tue, Dec 21, 2010 at 01:16:23AM -0600, Milton Miller wrote:
>>
>> [ Add Paul back to the CC list, and also Dipankar.
>>  Hopefully I killed the mime encodings correctly ]
>>
>> On Tue, 21 Dec 2010 at 08:48:50 +0900, Minchan Kim wrote:
>> > 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 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@csn.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 RCUlock 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.
>>
>> I'm not Paul but I can read the code in include/linux/rcuupdate.h.
>>
>> Holding rcu_read_lock_held isn't a problem, but using protected with
>> just the read lock is.
>>
>
> Bah, this was extremely careless of me as it's even written in teh
> documentation. In this specific case, it's simply allowed to ignore whether
> the RCU read lock is held or not and the BUG_ON check was unnecessary. The
> tree lock protects against parallel updaters which is what we really care
> about for using _protected.
>
> In a later cycle, I should look at reducing the RCU read lock hold time
> in migration. The main thing it's protecting is getting a stable
> reference to anon_vma and it's held longer than is necessary for that.

Yes.
I think if we want to reduce RCU read lock hold time, we should look
unmap_and_move in case of anon page.
After we hold a reference of anon_vma->external_refcount, anon_vma
would be stable so we can release rcu_read_unlock.
It can save many time.

> In the meantime, can anyone spot a problem with this patch?
>
> ==== 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: [<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@csn.ul.ie>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>

This is what I want.
Thanks.

-- 
Kind regards,
Minchan Kim

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

[Paul E. McKenney]

On Mon, Dec 20, 2010 at 05:01:46PM +0000, Mel Gorman 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@csn.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());
> 
> 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
> the RCU read lock not be held?

Apologies for the late reply!

It is OK to call rcu_dereference_protected() with rcu_read_lock() held,
but -only- if updates are somehow blocked -- for example, the treelock
being held as below.

It is OK to have extra protection, at least in this case.  ;-)

> > +	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;
> >  	}
> > 
> 
> -- 
> 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
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> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/

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

[Mel Gorman]

On Wed, Jan 12, 2011 at 03:21:13PM -0800, Paul E. McKenney wrote:
> On Mon, Dec 20, 2010 at 05:01:46PM +0000, Mel Gorman 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@csn.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());
> > 
> > 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
> > the RCU read lock not be held?
> 
> Apologies for the late reply!
> 
> It is OK to call rcu_dereference_protected() with rcu_read_lock() held,
> but -only- if updates are somehow blocked -- for example, the treelock
> being held as below.
> 
> It is OK to have extra protection, at least in this case.  ;-)
> 

Thanks for the clarification Paul.


-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

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

[KAMEZAWA Hiroyuki]

On Mon, 20 Dec 2010 15:23:36 +0000
Mel Gorman <mel@csn.ul.ie> 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@csn.ul.ie>

Thank you for fixing.
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>


> ---
>  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());
> +	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
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> 

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