[PATCH] memcg: remove mem_cgroup_reclaimable check from soft reclaim

[Vladimir Davydov <vdavydov@parallels.com>]

mem_cgroup_reclaimable() checks whether a cgroup has reclaimable pages
on *any* NUMA node. However, the only place where it's called is
mem_cgroup_soft_reclaim(), which tries to reclaim memory from a
*specific* zone. So the way how it's used is incorrect - it will return
true even if the cgroup doesn't have pages on the zone we're scanning.

I think we can get rid of this check completely, because
mem_cgroup_shrink_node_zone(), which is called by
mem_cgroup_soft_reclaim() if mem_cgroup_reclaimable() returns true, is
equivalent to shrink_lruvec(), which exits almost immediately if the
lruvec passed to it is empty. So there's no need to optimize anything
here. Besides, we don't have such a check in the general scan path
(shrink_zone) either.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
---
 mm/memcontrol.c |   43 -------------------------------------------
 1 file changed, 43 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 53393e27ff03..833b6a696aab 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1799,52 +1799,11 @@ int mem_cgroup_select_victim_node(struct mem_cgroup *memcg)
 	memcg->last_scanned_node = node;
 	return node;
 }
-
-/*
- * Check all nodes whether it contains reclaimable pages or not.
- * For quick scan, we make use of scan_nodes. This will allow us to skip
- * unused nodes. But scan_nodes is lazily updated and may not cotain
- * enough new information. We need to do double check.
- */
-static bool mem_cgroup_reclaimable(struct mem_cgroup *memcg, bool noswap)
-{
-	int nid;
-
-	/*
-	 * quick check...making use of scan_node.
-	 * We can skip unused nodes.
-	 */
-	if (!nodes_empty(memcg->scan_nodes)) {
-		for (nid = first_node(memcg->scan_nodes);
-		     nid < MAX_NUMNODES;
-		     nid = next_node(nid, memcg->scan_nodes)) {
-
-			if (test_mem_cgroup_node_reclaimable(memcg, nid, noswap))
-				return true;
-		}
-	}
-	/*
-	 * Check rest of nodes.
-	 */
-	for_each_node_state(nid, N_MEMORY) {
-		if (node_isset(nid, memcg->scan_nodes))
-			continue;
-		if (test_mem_cgroup_node_reclaimable(memcg, nid, noswap))
-			return true;
-	}
-	return false;
-}
-
 #else
 int mem_cgroup_select_victim_node(struct mem_cgroup *memcg)
 {
 	return 0;
 }
-
-static bool mem_cgroup_reclaimable(struct mem_cgroup *memcg, bool noswap)
-{
-	return test_mem_cgroup_node_reclaimable(memcg, 0, noswap);
-}
 #endif
 
 static int mem_cgroup_soft_reclaim(struct mem_cgroup *root_memcg,
@@ -1888,8 +1847,6 @@ static int mem_cgroup_soft_reclaim(struct mem_cgroup *root_memcg,
 			}
 			continue;
 		}
-		if (!mem_cgroup_reclaimable(victim, false))
-			continue;
 		total += mem_cgroup_shrink_node_zone(victim, gfp_mask, false,
 						     zone, &nr_scanned);
 		*total_scanned += nr_scanned;
-- 
1.7.10.4

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