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