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

[Mel Gorman <mel@csn.ul.ie>]

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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