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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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.  ;-)

> > +	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;
> >  	}
> > 
> 
> -- 
> Mel Gorman
> Part-time Phd Student                          Linux Technology Center
> University of Limerick                         IBM Dublin Software Lab
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