mm/debug_pagealloc.c: clean-up guard page handling code

mm/debug_pagealloc.c: clean-up guard page handling code

Patch series "Reduce memory waste by page extension user".

This patchset tries to reduce memory waste by page extension user.

First case is architecture supported debug_pagealloc.  It doesn't requires
additional memory if guard page isn't used.  8 bytes per page will be
saved in this case.

Second case is related to page owner feature.  Until now, if page_ext
users want to use it's own fields on page_ext, fields should be defined in
struct page_ext by hard-coding.  It has a following problem.

struct page_ext {
 #ifdef CONFIG_A
	int a;
 #endif
 #ifdef CONFIG_B
	int b;
 #endif
};

Assume that kernel is built with both CONFIG_A and CONFIG_B.  Even if we
enable feature A and doesn't enable feature B at runtime, each entry of
struct page_ext takes two int rather than one int.  It's undesirable waste
so this patch tries to reduce it.  By this patchset, we can save 20 bytes
per page dedicated for page owner feature in some configurations.

This patch (of 6):

We can make code clean by moving decision condition for set_page_guard()
into set_page_guard() itself.  It will help code readability.  There is no
functional change.

Link: http://lkml.kernel.org/r/1471315879-32294-2-git-send-email-iamjoonsoo.kim@lge.com
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>