mm: don't avoid high-priority reclaim on unreclaimable nodes

Commit 246e87a93934 ("memcg: fix get_scan_count() for small targets")
sought to avoid high reclaim priorities for kswapd by forcing it to scan
a minimum amount of pages when lru_pages >> priority yielded nothing.

Commit b95a2f2d486d ("mm: vmscan: convert global reclaim to per-memcg
LRU lists"), due to switching global reclaim to a round-robin scheme
over all cgroups, had to restrict this forceful behavior to
unreclaimable zones in order to prevent massive overreclaim with many
cgroups.

The latter patch effectively neutered the behavior completely for all
but extreme memory pressure.  But in those situations we might as well
drop the reclaimers to lower priority levels.  Remove the check.

Link: http://lkml.kernel.org/r/20170228214007.5621-6-hannes@cmpxchg.org
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Jia He <hejianet@gmail.com>
Cc: Mel Gorman <mgorman@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 014d0d181be0b8ed4136d9dfcb72d23a0052f820..2fd50ca88016307e94a62dac864bc1c2523334eb 100644 (file)
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2130,22 +2130,13 @@ static void get_scan_count(struct lruvec *lruvec, struct mem_cgroup *memcg,
        int pass;
 
        /*
-        * If the zone or memcg is small, nr[l] can be 0.  This
-        * results in no scanning on this priority and a potential
-        * priority drop.  Global direct reclaim can go to the next
-        * zone and tends to have no problems. Global kswapd is for
-        * zone balancing and it needs to scan a minimum amount. When
+        * If the zone or memcg is small, nr[l] can be 0. When
         * reclaiming for a memcg, a priority drop can cause high
-        * latencies, so it's better to scan a minimum amount there as
-        * well.
+        * latencies, so it's better to scan a minimum amount. When a
+        * cgroup has already been deleted, scrape out the remaining
+        * cache forcefully to get rid of the lingering state.
         */
-       if (current_is_kswapd()) {
-               if (!pgdat_reclaimable(pgdat))
-                       force_scan = true;
-               if (!mem_cgroup_online(memcg))
-                       force_scan = true;
-       }
-       if (!global_reclaim(sc))
+       if (!global_reclaim(sc) || !mem_cgroup_online(memcg))
                force_scan = true;
 
        /* If we have no swap space, do not bother scanning anon pages. */