mm/page_alloc.c: clean up check_for_memory()

check_for_memory() looks a bit confusing.  First of all, we have this:

if (N_MEMORY == N_NORMAL_MEMORY)
	return;

Checking the ENUM declaration, looks like N_MEMORY canot be equal to
N_NORMAL_MEMORY.

I could not find where N_MEMORY is set to N_NORMAL_MEMORY, or the other
way around either, so unless I am missing something, this condition will
never evaluate to true.  It makes sense to get rid of it.

Moving forward, the operations within the loop look a bit confusing as
well.

We set N_HIGH_MEMORY unconditionally, and then we set N_NORMAL_MEMORY in
case we have CONFIG_HIGHMEM (N_NORMAL_MEMORY != N_HIGH_MEMORY) and zone <=
ZONE_NORMAL.  (N_HIGH_MEMORY falls back to N_NORMAL_MEMORY on
!CONFIG_HIGHMEM systems, and that is why we can just go ahead and set
N_HIGH_MEMORY unconditionally)

Although this works, it is a bit subtle.

I think that this could be easier to follow:

First, we should only set N_HIGH_MEMORY in case we have CONFIG_HIGHMEM.
And then we should set N_NORMAL_MEMORY in case zone <= ZONE_NORMAL,
without further checking whether we have CONFIG_HIGHMEM or not.

Link: http://lkml.kernel.org/r/20180828210158.4617-1-osalvador@techadventures.net
Signed-off-by: Oscar Salvador <osalvador@suse.de>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Michael Hocko <mhocko@suse.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Pavel Tatashin <pavel.tatashin@microsoft.com
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 7e2fb6e38f340f2f1fe77f0fa9c7c60a27a0ba96..747031c2352db1a7fe51ddd2549af4a3215bdce8 100644 (file)
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6803,15 +6803,12 @@ static void check_for_memory(pg_data_t *pgdat, int nid)
 {
        enum zone_type zone_type;
 
-       if (N_MEMORY == N_NORMAL_MEMORY)
-               return;
-
        for (zone_type = 0; zone_type <= ZONE_MOVABLE - 1; zone_type++) {
                struct zone *zone = &pgdat->node_zones[zone_type];
                if (populated_zone(zone)) {
-                       node_set_state(nid, N_HIGH_MEMORY);
-                       if (N_NORMAL_MEMORY != N_HIGH_MEMORY &&
-                           zone_type <= ZONE_NORMAL)
+                       if (IS_ENABLED(CONFIG_HIGHMEM))
+                               node_set_state(nid, N_HIGH_MEMORY);
+                       if (zone_type <= ZONE_NORMAL)
                                node_set_state(nid, N_NORMAL_MEMORY);
                        break;
                }