tracing/user_events: Use alloc_pages instead of kzalloc() for register pages

kzalloc virtual addresses do not work with SetPageReserved, use the actual
page virtual addresses instead via alloc_pages.

The issue is reported when booting with user_events and
DEBUG_VM_PGFLAGS=y.

Also make the number of events based on the ORDER.

Link: https://lore.kernel.org/all/CADYN=9+xY5Vku3Ws5E9S60SM5dCFfeGeRBkmDFbcxX0ZMoFing@mail.gmail.com/
Link: https://lore.kernel.org/all/20220311223028.1865-1-beaub@linux.microsoft.com/

Cc: Beau Belgrave <beaub@linux.microsoft.com>
Reported-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

kernel/trace/trace_events_user.c

@@ -30,9 +30,10 @@
 
 /*
  * Limits how many trace_event calls user processes can create:
- * Must be multiple of PAGE_SIZE.
+ * Must be a power of two of PAGE_SIZE.
  */
-#define MAX_PAGES 1
+#define MAX_PAGE_ORDER 0
+#define MAX_PAGES (1 << MAX_PAGE_ORDER)
 #define MAX_EVENTS (MAX_PAGES * PAGE_SIZE)
 
 /* Limit how long of an event name plus args within the subsystem. */
@@ -1622,16 +1623,17 @@ static void set_page_reservations(bool set)
 
 static int __init trace_events_user_init(void)
 {
+	struct page *pages;
 	int ret;
 
 	/* Zero all bits beside 0 (which is reserved for failures) */
 	bitmap_zero(page_bitmap, MAX_EVENTS);
 	set_bit(0, page_bitmap);
 
-	register_page_data = kzalloc(MAX_EVENTS, GFP_KERNEL);
-
-	if (!register_page_data)
+	pages = alloc_pages(GFP_KERNEL | __GFP_ZERO, MAX_PAGE_ORDER);
+	if (!pages)
 		return -ENOMEM;
+	register_page_data = page_address(pages);
 
 	set_page_reservations(true);
 
@@ -1640,7 +1642,7 @@ static int __init trace_events_user_init(void)
 	if (ret) {
 		pr_warn("user_events could not register with tracefs\n");
 		set_page_reservations(false);
-		kfree(register_page_data);
+		__free_pages(pages, MAX_PAGE_ORDER);
 		return ret;
 	}