[PATCH 1/3] mm, page_alloc: Disable boosted watermark based reclaim on low-memory systems

[Mel Gorman]

An off-list bug report indicated that watermark boosting was affecting a
very small memory system and causing excessive reclaim. Watermark boosting
is intended to reduce the mixing of page mobility types within pageblocks
for high-order allocations. If the system has so little memory that pages
are not even grouped by mobility, then watermark boosting should also
be disabled.

Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
---
 mm/internal.h   | 6 +++++-
 mm/page_alloc.c | 6 ++++--
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/mm/internal.h b/mm/internal.h
index 3cf20ab3ca01..439561cc90d9 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -534,10 +534,14 @@ unsigned long reclaim_clean_pages_from_list(struct zone *zone,
 #endif
 #define ALLOC_KSWAPD		0x200 /* allow waking of kswapd */
 
+static inline void disable_watermark_boosting(void)
+{
+	watermark_boost_factor = 0;
+}
+
 enum ttu_flags;
 struct tlbflush_unmap_batch;
 
-
 /*
  * only for MM internal work items which do not depend on
  * any allocations or locks which might depend on allocations
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 3c4eb750a199..c5c05b6dc459 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5810,10 +5810,12 @@ void __ref build_all_zonelists(pg_data_t *pgdat)
 	 * made on memory-hotadd so a system can start with mobility
 	 * disabled and enable it later
 	 */
-	if (vm_total_pages < (pageblock_nr_pages * MIGRATE_TYPES))
+	if (vm_total_pages < (pageblock_nr_pages * MIGRATE_TYPES)) {
 		page_group_by_mobility_disabled = 1;
-	else
+		disable_watermark_boosting();
+	} else {
 		page_group_by_mobility_disabled = 0;
+	}
 
 	pr_info("Built %u zonelists, mobility grouping %s.  Total pages: %ld\n",
 		nr_online_nodes,
-- 
2.16.4