Re: [PATCH v6 20/37] mm: fix non-compound multi-order memory accounting in __free_pages

On Thu, Mar 21, 2024 at 04:48:53PM +0000, Matthew Wilcox wrote:

On Thu, Mar 21, 2024 at 09:36:42AM -0700, Suren Baghdasaryan wrote:

quoted

+++ b/mm/page_alloc.c

@@ -4700,12 +4700,15 @@ void __free_pages(struct page *page, unsigned int order)
 {
 	/* get PageHead before we drop reference */
 	int head = PageHead(page);
+	struct alloc_tag *tag = pgalloc_tag_get(page);
 
 	if (put_page_testzero(page))
 		free_the_page(page, order);
-	else if (!head)
+	else if (!head) {
+		pgalloc_tag_sub_pages(tag, (1 << order) - 1);
 		while (order-- > 0)
 			free_the_page(page + (1 << order), order);
+	}

Why do you need these new functions instead of just:

+	else if (!head) {
+		pgalloc_tag_sub(page, (1 << order) - 1);
		while (order-- > 0)
			free_the_page(page + (1 << order), order);
+	}

Actually, I'm not sure this is safe (I don't fully understand codetags,
so it may be safe).  What can happen is that the put_page() can come in
before the pgalloc_tag_sub(), and then that page can be allocated again.  
Will that cause confusion?