Re: [PATCH v2 07/11] mm: add find_get_entries_tag()

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

On Tue, Nov 17, 2015 at 09:42:22AM +1100, Dave Chinner wrote:
> On Fri, Nov 13, 2015 at 05:06:46PM -0700, Ross Zwisler wrote:
> > Add find_get_entries_tag() to the family of functions that include
> > find_get_entries(), find_get_pages() and find_get_pages_tag().  This is
> > needed for DAX dirty page handling because we need a list of both page
> > offsets and radix tree entries ('indices' and 'entries' in this function)
> > that are marked with the PAGECACHE_TAG_TOWRITE tag.
> > 
> > Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
> > ---
> >  include/linux/pagemap.h |  3 +++
> >  mm/filemap.c            | 61 +++++++++++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 64 insertions(+)
> > 
> > diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> > index a6c78e0..6fea3be 100644
> > --- a/include/linux/pagemap.h
> > +++ b/include/linux/pagemap.h
> > @@ -354,6 +354,9 @@ unsigned find_get_pages_contig(struct address_space *mapping, pgoff_t start,
> >  			       unsigned int nr_pages, struct page **pages);
> >  unsigned find_get_pages_tag(struct address_space *mapping, pgoff_t *index,
> >  			int tag, unsigned int nr_pages, struct page **pages);
> > +unsigned find_get_entries_tag(struct address_space *mapping, pgoff_t start,
> > +			int tag, unsigned int nr_entries,
> > +			struct page **entries, pgoff_t *indices);
> >  
> >  struct page *grab_cache_page_write_begin(struct address_space *mapping,
> >  			pgoff_t index, unsigned flags);
> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index d5e94fd..89ab448 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -1454,6 +1454,67 @@ repeat:
> >  }
> >  EXPORT_SYMBOL(find_get_pages_tag);
> >  
> > +/**
> > + * find_get_entries_tag - find and return entries that match @tag
> > + * @mapping:	the address_space to search
> > + * @start:	the starting page cache index
> > + * @tag:	the tag index
> > + * @nr_entries:	the maximum number of entries
> > + * @entries:	where the resulting entries are placed
> > + * @indices:	the cache indices corresponding to the entries in @entries
> > + *
> > + * Like find_get_entries, except we only return entries which are tagged with
> > + * @tag.
> > + */
> > +unsigned find_get_entries_tag(struct address_space *mapping, pgoff_t start,
> > +			int tag, unsigned int nr_entries,
> > +			struct page **entries, pgoff_t *indices)
> > +{
> > +	void **slot;
> > +	unsigned int ret = 0;
> > +	struct radix_tree_iter iter;
> > +
> > +	if (!nr_entries)
> > +		return 0;
> > +
> > +	rcu_read_lock();
> > +restart:
> > +	radix_tree_for_each_tagged(slot, &mapping->page_tree,
> > +				   &iter, start, tag) {
> > +		struct page *page;
> > +repeat:
> > +		page = radix_tree_deref_slot(slot);
> > +		if (unlikely(!page))
> > +			continue;
> > +		if (radix_tree_exception(page)) {
> > +			if (radix_tree_deref_retry(page))
> > +				goto restart;
> 
> That restart condition looks wrong. ret can be non-zero, but we
> start looking from the original start index again, resulting in
> duplicates being added to the return arrays...

This same restart logic is used in all the functions in this family:
find_get_entry() (though the tag is "repeat"), find_get_entries(),
find_get_pages(), find_get_pages_contig() and find_get_pages_tag().

Most don't have it well commented, but there is a good comment in
find_get_pages():

	if (radix_tree_exception(page)) {                               
		if (radix_tree_deref_retry(page)) {                     
			/*                                              
			 * Transient condition which can only trigger   
			 * when entry at index 0 moves out of or back   
			 * to root: none yet gotten, safe to restart.   
			 */                                             
			WARN_ON(iter.index);                            
			goto restart;                                   
		}   

I think the logic is correct, but I'm happy to add this comment in
find_get_entries_tag() if it would make things clearer.

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