[v3 PATCH 7/9] mm: vmscan: check if the demote target node is contended or not

[Yang Shi]

When demoting to the migration target node, the target node may have
memory pressure, then the memory pressure may cause migrate_pages()
fail.

If the failure is caused by memory pressure (i.e. returning -ENOMEM),
tag the node with PGDAT_CONTENDED.  The tag would be cleared once the
target node is balanced again.

Check if the target node is PGDAT_CONTENDED or not, if it is just skip
demotion.

Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
---
 include/linux/mmzone.h |  3 +++
 mm/vmscan.c            | 37 +++++++++++++++++++++++++++++++++++++
 2 files changed, 40 insertions(+)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 70394ca..d4e05c5 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -573,6 +573,9 @@ enum pgdat_flags {
 					 * many pages under writeback
 					 */
 	PGDAT_RECLAIM_LOCKED,		/* prevents concurrent reclaim */
+	PGDAT_CONTENDED,		/* the node has not enough free memory
+					 * available
+					 */
 };
 
 enum zone_flags {
diff --git a/mm/vmscan.c b/mm/vmscan.c
index fb931ded..9ec55d7 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1126,6 +1126,21 @@ static inline struct page *alloc_demote_page(struct page *page,
 }
 #endif
 
+static inline bool is_migration_target_contended(int nid)
+{
+	int node;
+	nodemask_t used_mask;
+
+
+	nodes_clear(used_mask);
+	node = find_next_best_node(nid, &used_mask, true);
+
+	if (test_bit(PGDAT_CONTENDED, &NODE_DATA(node)->flags))
+		return true;
+
+	return false;
+}
+
 static inline bool is_demote_ok(int nid, struct scan_control *sc)
 {
 	/* Just do demotion with migrate mode of node reclaim */
@@ -1144,6 +1159,10 @@ static inline bool is_demote_ok(int nid, struct scan_control *sc)
 	if (!has_migration_target_node_online())
 		return false;
 
+	/* Check if the demote target node is contended or not */
+	if (is_migration_target_contended(nid))
+		return false;
+
 	return true;
 }
 
@@ -1564,6 +1583,10 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 		nr_reclaimed += nr_succeeded;
 
 		if (err) {
+			if (err == -ENOMEM)
+				set_bit(PGDAT_CONTENDED,
+					&NODE_DATA(target_nid)->flags);
+
 			putback_movable_pages(&demote_pages);
 
 			list_splice(&ret_pages, &demote_pages);
@@ -2597,6 +2620,19 @@ static void shrink_node_memcg(struct pglist_data *pgdat, struct mem_cgroup *memc
 		 * scan target and the percentage scanning already complete
 		 */
 		lru = (lru == LRU_FILE) ? LRU_BASE : LRU_FILE;
+
+		/*
+		 * The shrink_page_list() may find the demote target node is
+		 * contended, if so it doesn't make sense to scan anonymous
+		 * LRU again.
+		 *
+		 * Need check if swap is available or not too since demotion
+		 * may happen on swapless system.
+		 */
+		if (!is_demote_ok(pgdat->node_id, sc) &&
+		    (!sc->may_swap || mem_cgroup_get_nr_swap_pages(memcg) <= 0))
+			lru = LRU_FILE;
+
 		nr_scanned = targets[lru] - nr[lru];
 		nr[lru] = targets[lru] * (100 - percentage) / 100;
 		nr[lru] -= min(nr[lru], nr_scanned);
@@ -3447,6 +3483,7 @@ static void clear_pgdat_congested(pg_data_t *pgdat)
 	clear_bit(PGDAT_CONGESTED, &pgdat->flags);
 	clear_bit(PGDAT_DIRTY, &pgdat->flags);
 	clear_bit(PGDAT_WRITEBACK, &pgdat->flags);
+	clear_bit(PGDAT_CONTENDED, &pgdat->flags);
 }
 
 /*
-- 
1.8.3.1