filemap: Add filemap_get_folios()

This is the equivalent of find_get_pages() but fills a folio_batch
instead of an array of pages.
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Christian Brauner (Microsoft) <brauner@kernel.org>

include/linux/pagemap.h

@@ -718,6 +718,8 @@ static inline struct page *find_subpage(struct page *head, pgoff_t index)
 	return head + (index & (thp_nr_pages(head) - 1));
 }
 
+unsigned filemap_get_folios(struct address_space *mapping, pgoff_t *start,
+		pgoff_t end, struct folio_batch *fbatch);
 unsigned find_get_pages_range(struct address_space *mapping, pgoff_t *start,
 			pgoff_t end, unsigned int nr_pages,
 			struct page **pages);

mm/filemap.c

@@ -2127,6 +2127,65 @@ unsigned find_lock_entries(struct address_space *mapping, pgoff_t start,
 	return folio_batch_count(fbatch);
 }
 
+/**
+ * filemap_get_folios - Get a batch of folios
+ * @mapping:	The address_space to search
+ * @start:	The starting page index
+ * @end:	The final page index (inclusive)
+ * @fbatch:	The batch to fill.
+ *
+ * Search for and return a batch of folios in the mapping starting at
+ * index @start and up to index @end (inclusive).  The folios are returned
+ * in @fbatch with an elevated reference count.
+ *
+ * The first folio may start before @start; if it does, it will contain
+ * @start.  The final folio may extend beyond @end; if it does, it will
+ * contain @end.  The folios have ascending indices.  There may be gaps
+ * between the folios if there are indices which have no folio in the
+ * page cache.  If folios are added to or removed from the page cache
+ * while this is running, they may or may not be found by this call.
+ *
+ * Return: The number of folios which were found.
+ * We also update @start to index the next folio for the traversal.
+ */
+unsigned filemap_get_folios(struct address_space *mapping, pgoff_t *start,
+		pgoff_t end, struct folio_batch *fbatch)
+{
+	XA_STATE(xas, &mapping->i_pages, *start);
+	struct folio *folio;
+
+	rcu_read_lock();
+	while ((folio = find_get_entry(&xas, end, XA_PRESENT)) != NULL) {
+		/* Skip over shadow, swap and DAX entries */
+		if (xa_is_value(folio))
+			continue;
+		if (!folio_batch_add(fbatch, folio)) {
+			unsigned long nr = folio_nr_pages(folio);
+
+			if (folio_test_hugetlb(folio))
+				nr = 1;
+			*start = folio->index + nr;
+			goto out;
+		}
+	}
+
+	/*
+	 * We come here when there is no page beyond @end. We take care to not
+	 * overflow the index @start as it confuses some of the callers. This
+	 * breaks the iteration when there is a page at index -1 but that is
+	 * already broken anyway.
+	 */
+	if (end == (pgoff_t)-1)
+		*start = (pgoff_t)-1;
+	else
+		*start = end + 1;
+out:
+	rcu_read_unlock();
+
+	return folio_batch_count(fbatch);
+}
+EXPORT_SYMBOL(filemap_get_folios);
+
 static inline
 bool folio_more_pages(struct folio *folio, pgoff_t index, pgoff_t max)
 {