mm/debug_pagealloc.c: don't allocate page_ext if we don't use guard page

What debug_pagealloc does is just mapping/unmapping page table.
Basically, it doesn't need additional memory space to memorize
something.  But, with guard page feature, it requires additional memory
to distinguish if the page is for guard or not.  Guard page is only used
when debug_guardpage_minorder is non-zero so this patch removes
additional memory allocation (page_ext) if debug_guardpage_minorder is
zero.

It saves memory if we just use debug_pagealloc and not guard page.

Link: http://lkml.kernel.org/r/1471315879-32294-3-git-send-email-iamjoonsoo.kim@lge.comSigned-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -607,6 +607,9 @@ static bool need_debug_guardpage(void)
 	if (!debug_pagealloc_enabled())
 		return false;
 
+	if (!debug_guardpage_minorder())
+		return false;
+
 	return true;
 }
 
@@ -615,6 +618,9 @@ static void init_debug_guardpage(void)
 	if (!debug_pagealloc_enabled())
 		return;
 
+	if (!debug_guardpage_minorder())
+		return;
+
 	_debug_guardpage_enabled = true;
 }
 
@@ -635,7 +641,7 @@ static int __init debug_guardpage_minorder_setup(char *buf)
 	pr_info("Setting debug_guardpage_minorder to %lu\n", res);
 	return 0;
 }
-__setup("debug_guardpage_minorder=", debug_guardpage_minorder_setup);
+early_param("debug_guardpage_minorder", debug_guardpage_minorder_setup);
 
 static inline bool set_page_guard(struct zone *zone, struct page *page,
 				unsigned int order, int migratetype)