mm: page_owner: fetch backtrace only for tracked pages

Very minor optimization.

Link: https://lkml.kernel.org/r/20210401212445.3534721-1-slyfox@gentoo.orgSigned-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_owner.c

@@ -147,15 +147,15 @@ void __reset_page_owner(struct page *page, unsigned int order)
 {
 	int i;
 	struct page_ext *page_ext;
-	depot_stack_handle_t handle = 0;
+	depot_stack_handle_t handle;
 	struct page_owner *page_owner;
 	u64 free_ts_nsec = local_clock();
 
-	handle = save_stack(GFP_NOWAIT | __GFP_NOWARN);
-
 	page_ext = lookup_page_ext(page);
 	if (unlikely(!page_ext))
 		return;
+
+	handle = save_stack(GFP_NOWAIT | __GFP_NOWARN);
 	for (i = 0; i < (1 << order); i++) {
 		__clear_bit(PAGE_EXT_OWNER_ALLOCATED, &page_ext->flags);
 		page_owner = get_page_owner(page_ext);