mm: optimise put_pages_list()

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.

Link: https://lkml.kernel.org/r/20211007192138.561673-1-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Anthony Yznaga <anthony.yznaga@oracle.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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);