Re: [PATCH 19/20] dax: Protect PTE modification on WP fault by radix tree entry lock

[Ross Zwisler <ross.zwisler@linux.intel.com>]

On Wed, Oct 19, 2016 at 09:25:05AM +0200, Jan Kara wrote:
> On Tue 18-10-16 13:53:32, Ross Zwisler wrote:
> > On Tue, Sep 27, 2016 at 06:08:23PM +0200, Jan Kara wrote:
> > > -	void *entry;
> > > +	void *entry, **slot;
> > >  	pgoff_t index = vmf->pgoff;
> > >  
> > >  	spin_lock_irq(&mapping->tree_lock);
> > > -	entry = get_unlocked_mapping_entry(mapping, index, NULL);
> > > -	if (!entry || !radix_tree_exceptional_entry(entry))
> > > -		goto out;
> > > +	entry = get_unlocked_mapping_entry(mapping, index, &slot);
> > > +	if (!entry || !radix_tree_exceptional_entry(entry)) {
> > > +		if (entry)
> > > +			put_unlocked_mapping_entry(mapping, index, entry);
> > 
> > I don't think you need this call to put_unlocked_mapping_entry().  If we get
> > in here we know that 'entry' is a page cache page, in which case
> > put_unlocked_mapping_entry() will just return without doing any work.
> 
> Right, but that is just an implementation detail internal to how the
> locking works. The rules are simple to avoid issues and thus the invariant
> is: Once you call get_unlocked_mapping_entry() you either have to lock the
> entry and then call put_locked_mapping_entry() or you have to drop it with
> put_unlocked_mapping_entry(). Once you add arguments about entry types
> etc., errors are much easier to make...

Makes sense.  You can add:

Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>

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