mm/huge_memory: try to free subpage in swapcache when possible

Subpages in swapcache won't be freed even if it is the last user of the
page until next time reclaim. It shouldn't hurt indeed, but we could try
to free these pages to save more memory for system.

Link: https://lkml.kernel.org/r/20220704132201.14611-12-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Zach O'Keefe <zokeefe@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/huge_memory.c

@@ -2538,7 +2538,7 @@ static void __split_huge_page(struct page *page, struct list_head *list,
 		 * requires taking the lru_lock so we do the put_page
 		 * of the tail pages after the split is complete.
 		 */
-		put_page(subpage);
+		free_page_and_swap_cache(subpage);
 	}
 }