[PATCH v2 2/3] mm/page_alloc.c: clear out zone->lowmem_reserve[] if the zone is empty

[Baoquan He]

When requesting memory allocation from a specific zone is not satisfied,
it will fall to lower zone to try allocating memory. In this case,
lower zone's ->lowmem_reserve[] will help protect its own memory resource.
The higher the relevant ->lowmem_reserve[] is, the harder the upper zone
can get memory from this lower zone.

However, this protection mechanism should be applied to populated zone,
but not an empty zone. So filling ->lowmem_reserve[] for empty zone is
not necessary, and may mislead people that it's valid data in that zone.

Node 2, zone      DMA
  pages free     0
        min      0
        low      0
        high     0
        spanned  0
        present  0
        managed  0
        protection: (0, 0, 1024, 1024)
Node 2, zone    DMA32
  pages free     0
        min      0
        low      0
        high     0
        spanned  0
        present  0
        managed  0
        protection: (0, 0, 1024, 1024)
Node 2, zone   Normal
  per-node stats
      nr_inactive_anon 0
      nr_active_anon 143
      nr_inactive_file 0
      nr_active_file 0
      nr_unevictable 0
      nr_slab_reclaimable 45
      nr_slab_unreclaimable 254

Here clear out zone->lowmem_reserve[] if zone is empty.

Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
---
 mm/page_alloc.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index c0c788798d8b..138a56c0f48f 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -7840,8 +7840,10 @@ static void setup_per_zone_lowmem_reserve(void)
 				idx--;
 				lower_zone = pgdat->node_zones + idx;
 
-				if (!sysctl_lowmem_reserve_ratio[idx]) {
+				if (!sysctl_lowmem_reserve_ratio[idx] ||
+				    !zone_managed_pages(lower_zone)) {
 					lower_zone->lowmem_reserve[j] = 0;
+					continue;
 				} else {
 					lower_zone->lowmem_reserve[j] =
 						managed_pages / sysctl_lowmem_reserve_ratio[idx];
-- 
2.17.2