[PATCH 2/3] mm/page_alloc: move the kswapd node skip check into a helper

[Joshua Hahn]

get_page_from_freelist() checks many conditions for each zone, often
with a big comment block explaining the conditions.

Move the kswapd check and its comment into should_skip_kswapd_zone() to
make the zone iteration cleaner to read.

While moving this, also resolve a checkpatch warning which complains of
a waitqueue_active() call with no corresponding comment explaining why
the lockless use is OK in this case.

No functional change intended.

Signed-off-by: Joshua Hahn <joshua.hahnjy@xxxxxxxxx>
---
 mm/page_alloc.c | 44 ++++++++++++++++++++++++++------------------
 1 file changed, 26 insertions(+), 18 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 4d829e5eac49d..2fe49f3c0de13 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3785,6 +3785,29 @@ static inline unsigned int alloc_flags_cma(gfp_t gfp_mask)
 	return ALLOC_DEFAULT;
 }
 
+/*
+ * If kswapd is already active on a node and the zone is below its kswapd
+ * watermark for this request, keep looking for other nodes that might be idle.
+ * This can happen if another process has NUMA bindings and is causing kswapd
+ * wakeups on only some nodes. Avoid accidental "node_reclaim_mode"-like
+ * behavior in this case.
+ */
+static bool should_skip_kswapd_zone(struct zone *zone, unsigned int order,
+				    const struct alloc_context *ac)
+{
+	unsigned long mark;
+
+	/* Lockless check, a stale result only affects zone selection */
+	if (waitqueue_active(&zone->zone_pgdat->kswapd_wait))
+		return false;
+
+	if (sysctl_numa_balancing_mode & NUMA_BALANCING_MEMORY_TIERING)
+		mark = promo_wmark_pages(zone);
+	else
+		mark = high_wmark_pages(zone);
+	return !zone_watermark_fast(zone, order, mark, ac->highest_zoneidx, 0);
+}
+
 /*
  * get_page_from_freelist goes through the zonelist trying to allocate
  * a page.
@@ -3796,9 +3819,6 @@ get_page_from_freelist(gfp_t gfp_mask, unsigned int order, int alloc_flags,
 	struct zoneref *z;
 	struct zone *zone;
 	struct pglist_data *last_pgdat = NULL;
-	enum zone_watermarks kswapd_watermark =
-		sysctl_numa_balancing_mode & NUMA_BALANCING_MEMORY_TIERING ?
-		WMARK_PROMO : WMARK_HIGH;
 	bool last_pgdat_dirty_ok = false;
 	bool no_fallback;
 	bool skip_kswapd_nodes = nr_online_nodes > 1;
@@ -3865,22 +3885,10 @@ get_page_from_freelist(gfp_t gfp_mask, unsigned int order, int alloc_flags,
 			}
 		}
 
-		/*
-		 * If kswapd is already active on a node and the zone is below
-		 * its kswapd watermark for this request, keep looking for
-		 * other nodes that might be idle. This can happen if another
-		 * process has NUMA bindings and is causing kswapd wakeups on
-		 * only some nodes. Avoid accidental "node_reclaim_mode"-like
-		 * behavior in this case.
-		 */
 		if (skip_kswapd_nodes &&
-		    !waitqueue_active(&zone->zone_pgdat->kswapd_wait)) {
-			mark = wmark_pages(zone, kswapd_watermark);
-			if (!zone_watermark_fast(zone, order, mark,
-					ac->highest_zoneidx, 0)) {
-				skipped_kswapd_nodes = true;
-				continue;
-			}
+		    should_skip_kswapd_zone(zone, order, ac)) {
+			skipped_kswapd_nodes = true;
+			continue;
 		}
 
 		cond_accept_memory(zone, order, alloc_flags);
-- 
2.53.1-Meta