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:

quoted

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 <redacted>
---
 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.

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>