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