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

Ross Zwisler
2015-11-17T18:10:36Z

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.