Commit 29c1f677 authored Jan 13, 2011 by Mel Gorman Committed by Linus Torvalds Jan 13, 2011

mm: migration: use rcu_dereference_protected when dereferencing the radix tree...


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
989f89c5 ("fix rcu_read_lock() in page
migraton").  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.

[akpm@linux-foundation.org: remove unneeded casts]
Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Cc: Minchan Kim <minchan.kim@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Milton Miller <miltonm@bga.com>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: <stable@kernel.org>		[2.6.37.early]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 22e5c47e

include/linux/radix-tree.h

+16 −0

Original line number	Diff line number	Diff line
		@@ -145,6 +145,22 @@ static inline void radix_tree_deref_slot(void *pslot)
		return rcu_dereference(*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

mm/migrate.c

+2 −2

Original line number	Diff line number	Diff line
		@@ -248,7 +248,7 @@ 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) {
		radix_tree_deref_slot_protected(pslot, &mapping->tree_lock) != page) {
		spin_unlock_irq(&mapping->tree_lock);
		return -EAGAIN;
		}
		@@ -320,7 +320,7 @@ 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) {
		radix_tree_deref_slot_protected(pslot, &mapping->tree_lock) != page) {
		spin_unlock_irq(&mapping->tree_lock);
		return -EAGAIN;
		}

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