mm/vmscan: Optimise shrink_page_list for non-PMD-sized folios

A large folio which is smaller than a PMD does not need to do the extra
work in try_to_unmap() of trying to split a PMD entry.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

mm/vmscan.c

@@ -1765,7 +1765,8 @@ static unsigned int shrink_page_list(struct list_head *page_list,
 			enum ttu_flags flags = TTU_BATCH_FLUSH;
 			bool was_swapbacked = PageSwapBacked(page);
 
-			if (unlikely(PageTransHuge(page)))
+			if (PageTransHuge(page) &&
+					thp_order(page) >= HPAGE_PMD_ORDER)
 				flags |= TTU_SPLIT_HUGE_PMD;
 
 			try_to_unmap(folio, flags);