mm: remove page->order

We already use page->private for storing the order of a page while it's in
the buddy allocator system; extend that to also storing the order while
it's in the pcp_llist.

Link: https://lkml.kernel.org/r/20250910142923.2465470-4-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Matthew Wilcox (Oracle)

include/linux/mm_types.h

@@ -97,10 +97,7 @@ struct page {
 				/* Or, free page */
 				struct list_head buddy_list;
 				struct list_head pcp_list;
-				struct {
-					struct llist_node pcp_llist;
-					unsigned int order;
-				};
+				struct llist_node pcp_llist;
 			};
 			struct address_space *mapping;
 			union {
@@ -111,7 +108,8 @@ struct page {
 			 * @private: Mapping-private opaque data.
 			 * Usually used for buffer_heads if PagePrivate.
 			 * Used for swp_entry_t if swapcache flag set.
-			 * Indicates order in the buddy system if PageBuddy.
+			 * Indicates order in the buddy system if PageBuddy
+			 * or on pcp_llist.
 			 */
 			unsigned long private;
 		};

mm/page_alloc.c

@@ -1520,7 +1520,7 @@ static void add_page_to_zone_llist(struct zone *zone, struct page *page,
 				   unsigned int order)
 {
 	/* Remember the order */
-	page->order = order;
+	page->private = order;
 	/* Add the page to the free list */
 	llist_add(&page->pcp_llist, &zone->trylock_free_pages);
 }
@@ -1549,7 +1549,7 @@ static void free_one_page(struct zone *zone, struct page *page,
 
 		llnode = llist_del_all(llhead);
 		llist_for_each_entry_safe(p, tmp, llnode, pcp_llist) {
-			unsigned int p_order = p->order;
+			unsigned int p_order = p->private;
 
 			split_large_buddy(zone, p, page_to_pfn(p), p_order, fpi_flags);
 			__count_vm_events(PGFREE, 1 << p_order);