[PATCH v2] mm: Optimise put_pages_list()

["Matthew Wilcox (Oracle)" <willy@infradead.org>]

Instead of calling put_page() one page at a time, pop pages off
the list if their refcount was too high and pass the remainder to
put_unref_page_list().  This should be a speed improvement, but I have
no measurements to support that.  Current callers do not care about
performance, but I hope to add some which do.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
v2:
 - Handle compound pages (Mel)
 - Comment why we don't need to handle PageLRU
 - Added call to __ClearPageWaiters(), matching that in release_pages()

 mm/swap.c | 23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/mm/swap.c b/mm/swap.c
index af3cad4e5378..9f334d503fd2 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -134,18 +134,27 @@ EXPORT_SYMBOL(__put_page);
  * put_pages_list() - release a list of pages
  * @pages: list of pages threaded on page->lru
  *
- * Release a list of pages which are strung together on page.lru.  Currently
- * used by read_cache_pages() and related error recovery code.
+ * Release a list of pages which are strung together on page.lru.
  */
 void put_pages_list(struct list_head *pages)
 {
-	while (!list_empty(pages)) {
-		struct page *victim;
+	struct page *page, *next;
 
-		victim = lru_to_page(pages);
-		list_del(&victim->lru);
-		put_page(victim);
+	list_for_each_entry_safe(page, next, pages, lru) {
+		if (!put_page_testzero(page)) {
+			list_del(&page->lru);
+			continue;
+		}
+		if (PageHead(page)) {
+			list_del(&page->lru);
+			__put_compound_page(page);
+			continue;
+		}
+		/* Cannot be PageLRU because it's passed to us using the lru */
+		__ClearPageWaiters(page);
 	}
+
+	free_unref_page_list(pages);
 }
 EXPORT_SYMBOL(put_pages_list);
 
-- 
2.32.0