[RFC] 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 there are other refcounts and pass the remainder
to free_unref_page_list().  This should be a speed improvement,
but I have no measurements to support that.  It's also not very
widely used today, so I can't say I've really tested it.  I'm only
bothering with this patch because I'd like the IOMMU code to use it
https://lore.kernel.org/lkml/20210930162043.3111119-1-willy@infradead.org/

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 mm/swap.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/mm/swap.c b/mm/swap.c
index af3cad4e5378..f6b38398fa6f 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -139,13 +139,14 @@ EXPORT_SYMBOL(__put_page);
  */
 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);
 	}
+
+	free_unref_page_list(pages);
 }
 EXPORT_SYMBOL(put_pages_list);
 
-- 
2.32.0