linux-ext4 - Re: [PATCH 2/6] mm: Invalidate DAX radix tree entries only if appropriate

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Date:   Fri, 9 Dec 2016 13:02:33 +0100
From:   Jan Kara <jack@...e.cz>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Jan Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.org,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        linux-ext4@...r.kernel.org, linux-mm@...ck.org,
        linux-nvdimm@...ts.01.org
Subject: Re: [PATCH 2/6] mm: Invalidate DAX radix tree entries only if
 appropriate

Hi,

On Wed 30-11-16 09:08:41, Jan Kara wrote:
> > > +static int __dax_invalidate_mapping_entry(struct address_space *mapping,
> > > +					  pgoff_t index, bool trunc)
> > > +{
> > > +	int ret = 0;
> > > +	void *entry;
> > > +	struct radix_tree_root *page_tree = &mapping->page_tree;
> > > +
> > > +	spin_lock_irq(&mapping->tree_lock);
> > > +	entry = get_unlocked_mapping_entry(mapping, index, NULL);
> > > +	if (!entry || !radix_tree_exceptional_entry(entry))
> > > +		goto out;
> > > +	if (!trunc &&
> > > +	    (radix_tree_tag_get(page_tree, index, PAGECACHE_TAG_DIRTY) ||
> > > +	     radix_tree_tag_get(page_tree, index, PAGECACHE_TAG_TOWRITE)))
> > > +		goto out;
> > > +	radix_tree_delete(page_tree, index);
> > 
> > You could use the new __radix_tree_replace() here and save a second
> > tree lookup.
> 
> Hum, I'd need to return 'node' from get_unlocked_mapping_entry(). So
> probably I'll do it in a patch separate from this fix. But thanks for
> suggestion.

So I did this and quickly spotted a problem that when you use
__radix_tree_replace() to clear an entry, it will leave tags for that entry
set and that results in surprises. So I think I'll leave the code with
radix_tree_delete() for now.

It would probably make sense to make __radix_tree_replace() to clear tags
when we replace entry with NULL or at least WARN if some tags are set...
What do you think?

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR
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