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

[Milton Miller <miltonm@bga.com>]

[ 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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