[RFC] find_get_heads_contig

[Matthew Wilcox <willy@infradead.org>]

I was going to convert find_get_pages_contig() to only return head pages,
but I want to change the API to take a pagevec like the other find_*
functions have or will have.  And it'd be nice if the name of the function
reminded callers that it only returns head pages.  So comments on this?

diff --git a/mm/filemap.c b/mm/filemap.c
index 31ba06409dfa..b7dd2523fe79 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2093,6 +2093,58 @@ unsigned find_get_pages_contig(struct address_space *mapping, pgoff_t index,
 }
 EXPORT_SYMBOL(find_get_pages_contig);
 
+/**
+ * find_get_heads_contig - Return head pages for a contiguous byte range.
+ * @mapping: The address_space to search.
+ * @start: The starting page index.
+ * @end: The final page index (inclusive).
+ * @pvec: Where the resulting pages are placed.
+ *
+ * find_get_heads_contig() will return a batch of head pages from
+ * @mapping.  Pages are returned with an incremented refcount.  Only the
+ * head page of a THP is returned.  In contrast to find_get_entries(),
+ * pages which are partially outside the range are returned.  The head
+ * pages have ascending indices.  The indices may not be consecutive,
+ * but the bytes represented by the pages are contiguous.  If there is
+ * no page at @start, no pages will be returned.
+ *
+ * Return: The number of pages which were found.
+ */
+unsigned find_get_heads_contig(struct address_space *mapping, pgoff_t start,
+		pgoff_t end, struct pagevec *pvec)
+{
+	XA_STATE(xas, &mapping->i_pages, start);
+	struct page *page;
+
+	rcu_read_lock();
+	for (page = xas_load(&xas); page; page = xas_next(&xas)) {
+		if (xas.xa_index > end)
+			break;
+		if (xas_retry(&xas, page) || xa_is_sibling(page))
+			continue;
+		if (xa_is_value(page))
+			break;
+
+		if (!page_cache_get_speculative(page))
+			goto retry;
+
+		/* Has the page moved or been split? */
+		if (unlikely(page != xas_reload(&xas)))
+			goto put_page;
+
+		if (!pagevec_add(pvec, page))
+			break;
+		continue;
+put_page:
+		put_page(page);
+retry:
+		xas_reset(&xas);
+	}
+	rcu_read_unlock();
+	return pagevec_count(pvec);
+}
+EXPORT_SYMBOL(find_get_heads_contig);
+
 /**
  * find_get_pages_range_tag - Find and return head pages matching @tag.
  * @mapping:	the address_space to search